Male deity from the Gundestrup Cauldron, Denmark https://www.worldhistory.org/image/13359/celtic-deity-gundestrup-cauldron/ As if the Picts weren't mysterious enough already, one area in which absolutely no record survives (perhaps because we have no texts written in the Pictish language) is their mythology. While Christianization has led to the demise of many indigenous mythologies, in some places they were at least partially preserved, often ironically by monks (and poets in the case of Iceland). As with Anglo-Saxon mythology (which we know at least some details about unlike that of the Picts), we are forced to look to the mythologies of their neighbors to get an idea of what it might have been like. This is an exercise in which caution is needed, as there is no guarantee that the same gods were worshipped by different peoples no matter how similar they were.
This is already the case between Gaelic and Welsh mythology for instance, where there is some overlap, but sometimes gods are known in one and not the other (such as Brigit, a Gaelic goddess who has no Welsh equivalent with a similar name). I will therefore focus on shared attributes between Gaelic and Welsh (and sometimes Gaulish) versions rather than digressing too far into details of the variants (except where they are the only myth known about a particular god).
Here I will list those gods which seem to have been worshipped across the Celtic world ('pan-Celtic'), as it seems clear from the linguistic evidence we have from place names and personal names that the Picts were Celts. In most cases, their names will resemble the Welsh forms (as the Pictish language seems to have been most closely related to Welsh) and will also have applied to the Brythons of Southern Scotland, whose mythology would have been even closer to that of the Welsh. Explanation of the meaning of their names will only be given where the etymology is reasonably secure, as I don't wish to speculate on uncertain etymologies. 1. Lú
Three-headed altar from Reims, France, though to represent Lugos https://commons.wikimedia.org/wiki/File:Autel_tricephale_MuseeStRemi_Reims
One of the most commonly attested Celtic deities, he was known as 'Lugos' in Ancient Celtic and 'Lleu' in Welsh (pronouced like 'lay', with the 'll' pronounced by positioning your tongue to make an 'l' sound, but blowing through instead). I've chosen the Irish form because so far we have no evidence that the 'll' sound in Welsh was present in the Pictish language (also for the sake of easier reading and pronunciation).
While so far we have no Roman shrines dedicated to 'Lugus Mercurius' or the like, it seems fair to assume that the Romans would've equated Lú with Mercury, as Caesar referred to 'Gaulish Mercury' as: “inventor of all the arts”1, which corresponds to Lú's epithet samildánach, meaning “skilled in all the arts”. Mercury also seems to have been the most popular god across the Romano-Celtic world, so it seems most likely he had been syncretized with a local deity (possibly multiple).
The details of Lú's life in Irish and Welsh mythology differ, but a common thread is that he was raised by his uncle rather than his parents, as they were not married in either mythos. In Irish mythology, his uncle is Goibniu, the smith god, but variations say he was raised by the sea god, Mannanán MacLir (who seems not to have been his relative at all).
In Welsh he was called Lleu Llaw Gyffes, which supposedly means 'fair one of the skillful hand', a name given unwittingly to him by his mother, Aranrhod, in reference to the skill with which he killed a wren (presumably with a stone). This parallels his epithet in Gaelic, Lámfada, meaning 'of the long arm', referring to his use of a sling to kill his grandfather, Balor, King of the Fomorians. In either case he was certainly associated with stone-throwing, whether with hand or sling.
However, his Irish variant also references his skills as a carpenter, a blacksmith, a warrior/hero, a musician, a scholar, a poet and a magician and thus gained entrance to the court of the Tuath Dé ('Tribe of the Gods', more commonly known as the 'Tuatha dé Danaan') by asking the guard if any of the other gods had all these talents. Thus, his epithet of Samildánach referred to his many talents. The Welsh Fourth Branch of the Mabinogi notes that he was particularly sturdy from babyhood, so he embodied both physical and mental excellence.
However, one skill in particular that may have had more myths surrounding it (aside from stone-throwing) is shoemaking. In Osma, Spain, he was invoked for the sake of a shoemakers' guild, which has been compared to his role as a shoemaker in the Fourth Branch2, where his uncle and foster-father, Gwydion, magically fashions leather out of seaweed and dulse from which he and Lleu create shoes for Lleu's mother (this is the same episode in which she named him). This particular talent may also have connected him with the mythical Irish leprechaun, which is thought to represent a diminutive, Christianized form of Lú3. 2. Nudd
The Tandragee Idol from County Armagh, Northern Ireland, thought to represent Nuada
Known as 'Nuada' in Gaelic (the 'd' in both names pronounced as 'th' in 'the'), he was worshipped in Britain most likely as *Nodens (a reconstructed form based on the dative 'Nodenti' on inscriptions, a god most often equated with Mars, but sometimes with Silvanus (Roman Silenus, companion of Dionysus); showing that Celtic gods often didn't correspond exactly with Roman ones. Both Nudd and Nuada are known to have had silver hands, the former possessing the epithet Llaw Ereint and the latter Airgetlám which refer to this.
While we have no tales regarding Nudd's silver hand, for Nuada we get more detail that he lost his hand (or arm) in battle with an enemy tribe (the Fir Bolg). This made him ineligible for kingship, as a king of the Tuath Démust have been physically perfect. He was replaced by Bres, who was son of the Tuath Déprincess Ériu (after whom Ireland is named) and Elatha, a prince of the Fomorians (another enemy tribe).
Bres was beautiful, but oppressed the gods (a similar note is made about Loki in Norse mythology, that he is beautiful but wicked). The god of healing, Dian Cécht (Nuada's brother), and god of precious metalworking, Credne, made Nuada a new hand/arm of silver, which allowed him to retake the kingship of the gods.
The loss of his hand/arm also makes Nudd comparable to Týr from Norse mythology, who is a war god like Mars. The association of *Nodens with Mars makes it very likely that Nudd was a war god, though his additional association with Silvanus gives him another potential role as protector of forests and patron of hunting.
This may mean than Nudd is the same as Cocidios, another local deity attested in Roman inscriptions from Northern England and who was also identified with both Mars and Silvanus. He was sometimes depicted with horns, which can be compared with the Tandragee Idol from County Armagh, Northern Ireland, which is thought to depict Nuada. Votive offerings of dog statues at *Nodens' shrine at Lydney Park, Gloucestershire, suggest an association with healing4. 3. Taran
Idol of Jupiter Taranis, from Seltz, France https://commons.wikimedia.org/wiki/File:Jupiter-Taranis-Seltz_%283%29.jpg
Known as 'Tuireann' in Gaelic and 'Taranis' in Ancient Celtic, his name means 'thunderer' and he was predictably equated with Jupiter by the Romans. Curiously, 'Taran' is only known as a personal name in Welsh and absolutely no Welsh myths survive about him and he only plays a marginal role in Irish mythology. His Sons kill Lú's father, Cian (a physician god) and Lú forces them to perform a series of near impossible tasks (much like Hercules) to atone for their crime.
One of these is to steal a magical pigskin, into which any liquid poured will become a healing elixir. Another was to shout three times from a hilltop owned by a king and his three princes, among whom Cian had been fostered. Attempting this last task, the king and his princes fought to prevent the Sons of Tuireann from shouting from the hilltop (as they had murdered Cian) and the Sons were mortally wounded. Tuireann finally appears to beg Lú to give them the healing elixir from the magical pigskin, but Lú, bent on revenge for his father's murder, refuses and they die. 4. Brigit
Idol of a Celtic goddess thought to represent Brigantia, from Ménez-hom, France
Known as 'Bride' in Scotland and 'Brigantia' in Ancient Celtic, Brigit is another deity absent from Welsh mythology. In Irish mythology she is a goddess of poets and wisdom, but is also split into three sisters, the other two presiding over healing and smithing. Thus, somewhat like Lú, she was a multifaceted goddess with many attributes.
The Romans equated her with Minerva, who, like her Greek counterpart Athena, was probably conflated with Brigantia because of her association with wisdom. Brigantia was also conflated with other minor Roman goddesses, such as Fortuna and Victoria (I shouldn't need to explain what functions they had). Brigantia actually had a shrine in Scotland at the Roman fort of Birrens, Dumfriesshire. She gave her name to the Brigantes, a Brythonic tribe in Northern England.
To the Gaels, Brigit also governed divination and prophecy and was invoked by women for help in giving birth. However, she is commonly known in her Christian guise, St Brigid of Kildare, who might have been a historical person (but even if she was, elements of her story are likely taken from her pagan counterpart).
St Brigid was a virgin who had a larder always filled with food, her cows provided a loch of milk and one measure of her malt made enough ale for seventeen churches5. In Scottish mythology, Bride is saved by Angus from the clutches of his mother, the Cailleach Bheurra, goddess of winter, at the beginning of February. Her rescue heralds spring and was marked by the festival of Imbolc. A similar theme can be found in the Greek myth of Persephone. 5. Dôn
Little is known about this goddess (of whom 'Danu' is the Gaelic form of her name), other than that she is the mother of all the gods in both Irish and Welsh mythology (the Tuatha dé Danaan are named after her). She is thought to have given her names to rivers such as the Danube in Central Europe and the Don in Russia, but there are no surviving myths about her and she remains enigmatic. In Welsh mythology she and Math (mythical King of Gwynedd) are the children of Mathonwy, another mysterious figure about whom nothing other than the name is known.
Known in Gaelic as 'Goibniu', in Welsh as 'Gofannon' and Gaulish as 'Gobannos'; I've chosen the name 'Govan' to reflect a compromise between the Gaelic and Welsh forms (both the 'b' and 'f' are pronounced like 'v' and it's unclear where the extra 'on' in 'Gofannon' comes from). In any case it means 'smith' and that is what he was god of; the district of Govan in Glasgow comes from the Gaelic Baile a' Ghobhainn meaning 'smiths' town' and since it is also the former capital of the Kingdom of Strathclyde, I find it fitting that this neighborhood should share its name with a god once worshipped by its inhabitants' ancestors. Not much is really to be said of this god, other than that he did what he was best at in various tales.
Image of Ogma on the Library of Congress, Washington D.C., USA https://garystockbridge617.getarchive.net/amp/media/library-of-congress-john-adams-building-figure-of-ogma-on-door-library-of-congress
Known as 'Ogmios' in Gaulish, he is another figure absent from Welsh mythology. He is an interesting god being both renowned as a warrior, but also eloquent and inventor of writing. The 2nd century Greek writer, Lucian of Samosata, claimed that the Gauls depicted Ogmios with a bow and club like Hercules, but unlike Hercules he was depicted as a bald old man, trailed by a band of happy men in chains through their ears which linked them to Ogmios' tongue (representing his persuasive ability).
In Irish mythology, he is the champion of the Tuath Dé, but is challenged (and defeated) by Lú. He was also held to be the inventor of ogham, a type of alphabet with letters often named after trees and likely originating in Munster in South-West Ireland. In being depicted as a martial old man by the Gauls and inventor of writing by the Gaels, he resembles Odin from Norse mythology, but in all other respects they are different.
Painting of Angus Óg by John Duncan
This Welsh name means 'Divine Youth/Son' and is thus the same as an epithet given to Angus, Gaelic god of love and youth: Mac ind Óc. The Pictish equivalent of 'Angus' seems to have been 'Onuist/Unuist' and the god may have been known to the Picts by this name. Yet among the Brythons he was 'Mabon', earlier 'Maponos'. He was equated by the Romans with Apollo and was thus associated with music and poetry.
However, in both Irish and Welsh mythology, he was a warrior, including being one of Arthur's companions and as mentioned above, in Scottish mythology he is the rescuer of Brigit from his mother the Cailleach (which would imply that the Scottish Cailleach and Welsh Modron are the same, as he was known in Wales as 'Mabon ap ('son of') Modron').
It is also in Scotland however where his Apollo-like attributes were emphasized, as he is said to own a golden harp with silver strings, enchanting and drawing youths to him in the woods. His kisses would then become birds that would follow those he has kissed home singing love songs and whispering memories of the youths' time with him in their ears. He is also said to be the source of echo, as his mother punished him for disobeying her and trying to become King of the Universe by trapping him in rocks and forcing him to repeat the words of others6.
Wooden statue of Mannanán MacLir at Downhill, County Derry, Northern Ireland https://www.geograph.ie/photo/4021842
Known in Gaelic as 'Mannanán MacLir ('fab Llýr' in Welsh), Manawydan is god of the sea ('Lir/ Llýr' means 'sea', so he is 'son of the sea', but is also known in Gaelic as MacAlloit/Alloid, which means 'son of the land'7). Nothing apart from his name associates him with the sea in Welsh mythology, but in Irish mythology he rides a horse-drawn chariot across the waves and wears a cloak that can change color according to light as the sea does.
Both Irish and Welsh versions are magicians and craftsmen and Mannanán gives Lú magical gifts to help defeat the Fomorians, including a boat that is controlled by thought and needs no oars or sails, a horse which can ride on land or water and a sword called 'Fragarach' which can pierce any armor8.
In Welsh mythology he and his companion, Pryderi become leatherworkers, but they are driven out of a town by local leatherworkers because they were running them out of business, so talented were Manawydan and Pryderi at the craft. However, the fact that there seem to be no Roman inscriptions dedicated to a Celtic Neptune (with the possible exception of the Chichester Stone) perhaps suggests that he was not a popular god. 10.Math
Idol of Sucellos from Sauvat, France https://commons.wikimedia.org/wiki/File:Sucellus_MAN_St_Germain.jpg
This last one is admittedly a guess, as Welsh mythology is notoriously heavily Christianized and it can be hard to discern how the attributes of its characters correspond to pagan gods. Nevertheless, I feel there is a strong enough case for Math fab Mathonwy, King of Gwynedd in the Fourth Branch, to correspond with the Irish god known as 'the Dadga' ('good god') and the Gallo-British god 'Sucellos' ('Sucellos' is thought to mean 'striker', which is possibly also the meaning of one of the Dagda's epithets: Cerrce) .
The main thing Math and the Dadga have in common is that they are powerful magicians; seemingly the most powerful in their respective mythologies; indeed, one of the Dagda'a epithet's is Eochaid Ollathair, which means 'horseman allfather'9, much like Odin. Also like Odin, they act as kings of the other gods and lead their warriors into battle, but Miranda Green notes that Math's reliance on a virgin's lap to rest his feet on and him magically turning his nephews into male and female animals means he may have been associated with fertility.
The Dagda is similar in his fertility association for he mated with different goddesses and possessed a cauldron of plenty (another of his epithets, Dáire, means 'the fertile one'10). Both the cauldron and his magical club (with which he could both take and restore life) make him readily identifiable with Sucellos, who is often depicted with a hammer and a pot and was associated with winemaking. The Romans identified him with Silvanus, which probably meant his fertility attributes were what was most apparent to them.
1Caesar, The Gallic War: 6:17
3Cotterell & Storm, 1999: The ultimate Encyclopedia of Mythology, p.145
5Green, 1992: Dictionary of Celtic Myth and Legend, pp.50-51
8Green, 1992: Dictionary of Celtic Myth and Legend, p.139
10Green, 1992: Dictionary of Celtic Myth and Legend, p.145