Dylan ail Don

In the Fourth Branch of the Mabinogi Dylan is born to Arianrhod in a miraculous narrative context connected to Math ap Mathonwy's court. When the newborn is baptised he plunges into the sea and 'immediately when he was in the sea, he took its nature'; he is recognised by Math and by his maternal kin (Arianrhod is daughter of Dôn), situating him within the household/divine family of Dôn. His birth and instant affinity with seawater mark him as a personified aspect of the sea rather than a mere human child.

The medieval account describes Dylan initially as a 'sturdy blonde boy' at birth but gives no sustained anthropomorphic detail. The decisive descriptive moment is his immersion in the sea at baptism after which he 'took its nature, and swam as well as the best fish that was therein.' Later modern summaries sometimes call him a sea-god or merman, but the primary narrative confines the depiction to a human infant who immediately manifests fully aquatic nature upon contact with seawater.

The primary attested ability is an innate and immediate aquatic adaptation: upon entering the sea Dylan 'took its nature' and swam as well as any fish. The narrative presents no catalogue of additional powers, cult, or ongoing intervention in human affairs; his status in modern summaries as a 'sea-god' is interpretive. His narrative is also bounded by mortality within the tale: he is struck and killed by his uncle Gofannon, showing that his marine nature does not render him invulnerable.