DYLAN: [Welsh, ocean, wave]. Welsh aquatic hero or sea demigod, the son of Arianrhod, daughter of Dôn in the fourth branch of the Mabinogi; may carry the epithet Ail Ton, Eil Ton, Eil Tôn, Eilton, Eil Don [Welsh, son of wave] or Ail Mor [Welsh, son of the sea]. Described as dark, Dylan contrasts with his fair twin brother, Lleu Llawgyffes.
Arianrhod gives birth to Dylan immediately after stepping over Gwydion’s magic wand. He takes to the sea as soon as he is baptized, and assumes the sea’s nature; he can swim as well as any fish and no wave ever breaks under him. He is killed by a single blow from his uncle, Gofannon fab Dôn. The rising tide rises to avenge the killing; the roar of the tide at the mouth of the Conway River is thought to be the death-groan of Dylan. His name is also cited in the Book of Taliesin and the Triads. Learned speculation asserts that Dylan may be based on an independent sea-divinity whose story became associated with Lleu Llaw Gyffes…. Dylan may have served as a model for the Arthurian figure Dyonas, the father of the beautiful Vivien/Vivian.
– James MacKillop, Oxford Dictionary of Celtic Mythology, 162
… Then Gwydion son of Don took his magician’s rod and held it down low.
“Step across this,” he said [to Arianrhod], “and if you are a maiden, I will know it.”
She stepped over the rod then, and as she did she dropped a big, fine, yellow-haired boy. What the boy did was to utter a loud cry. After the boy’s cry, she made for the door, dropping as she went some little thing from her. Before anyone could have a second glance at it, Gwydion took it, wrapped a brocaded silk coverlet around it and hid it. Where he did it was in a small chest at the foot of his bed.
“Well,” said Math son of Mathonwy, “I will have this one baptized,” he said, looked at the sturdy yellow-haired boy. “This name I will give him is Dylan.”
The boy was baptized, and as soon as he was, he made for the sea. No sooner had he come to the sea then he took the sea’s nature, and swam as well as the best fish in the sea; because of that he was called Dylan Eil Ton. No wave ever broke under him; the blow that caused his death was delivered by Gofannon his uncle, and that was one of the three unfortunate blows.
– “Math son of Mathonwy,” in The Mabinogi and Other Medieval Welsh Tales, tr. by Patrick Ford, 98-99