Maelgwn ap Cadwallon (c.480-c.547, reigned from 520s?) (Latin: Maglocunus; English: Malcolm), also known as Maelgwn Gwynedd and Maelgwn Hir ('the Tall'), was king of Gwynedd, and a character from Celtic mythology.

The historical Maelgwn was one of the most influential rulers of 6th century Britain, and has become one of the most famous (or infamous) leaders in Welsh history. The Christian writer Gildas (who referred to him Malgocunus, meaning 'Great Hound') attacked him in De Excidio Britanniae as 'first in evil,' one of the most vicious tyrants in British history, and accused him of having murdered his uncle while still a youth. Geoffrey of Monmouth (calling him Malgo) specifically noted his handsome features, but also claimed that he was homosexual. Other notable misdeeds said to have been performed by Maelgwn include murdering his first wife and his nephew in order to clam his nephew's wife as his own. Maelgwn remained rather unpopular with leading church writers, despite several attempts at patronage, including founding the bishopric of Bangor.

He was also said to be a great patron of the arts and a skilled lawgiver, although some attribute this reputation to Maelgwn's own propaganda. He established court at Deganwy, and surrounded himself with an entourage of bards and artisans who wrote glowingly of his achievements. By the time of his death, Maelgwn had established himself as the preeminent ruler of the region, and his sons Rhun and Brude would inherit control over both Gwynedd and the lands of the Picts in southern Britain.

Elias Gruffydd preserved the following mythological tradition in a manuscript he wrote in the mid-16th century, although some critics believe this story is much older.

King Maelgwn demanded that a distant son of one of his lords, Elphin, praise him and his court. Elphin refused, claiming his bard, Taliesin was a better bard and his wife a prettier woman than anyone the King had in his court. Taliesin knew what was happening, because he was a seer, and told Elphin's wife. Maelgwn's son Rhun went to Elphin's house to seduce his wife and prove Elphin's claims weren't true. Rhun got her drunk. When she passed out, Rhun tried to take her wedding ring off to prove her unfaithfulness; since the ring wouldn't come off, he cut off her finger. When King Maelgwn attempted to show the finger to Elphin, he pointed out that his wife cut her fingernails more often than the owner of the finger, had servants to kneed dough and never had any under her nails, and her ring was loose on her finger, and that one was tight.

Maelgwn demanded Taliesin come to his court to prove the other claim wrong. Taliesin gave twenty minutes for both himself and the King's bards to come up with an epic. The royal bards couldn't do it. When it came Taliesin's time, he caused a massive wind to rattle the castle. Frightened, Maelgwn sent for Elphin. Taliesin's next song caused Elphin's chains to detach. Maelgwn challenged the pair to a horse race. Taleisin arrived the next day with an old, weak horse. As each of the king's horses passed him at the very start of the race, Taliesin touched its rump with a twig of holly. When they had all passed, he dropped his hat to the ground. When the king's horses came back, right before the finish line, they stopped at the holly twigs Taliesin had laid there, and began to dance. Taliesin's old horse strolled back in quite a bit later and won the race.

The 1911 Britannica has this tale to tell of him...

The first Eisteddfod of which any account seems to have descended to us was one held on the banks of the Conway in the 6th century, under the auspices of Maelgwn Gwynedd, prince of North Wales. Maelgwn on this occasion, in order to prove the superiority of vocal song over instrumental music, is recorded to have offered a reward to such bards and minstrels as should swim over the Conway. There were several competitors, but on their arrival on the opposite shore the harpers found themselves unable to play owing to the injury their harps had sustained from the water, while the bards were in as good tune as ever.