In a Welsh Myrddin poem known as The Prophecy of Myrddin and Gwenddydd, Gwenddydd, Myrddinís sister, says, "I ask my Llallogan Myrddin, a wise man, a prophet". Llallogan is general translated as "twin brother". It is possible Lailoken was merely a nickname for Merlin, as Lailoken resembles the Welsh word for a twin and Merlin was thought to have a twin sister, Ganieda.
Lailokenís legend is very similar to Myrddinís: Lailoken was said to have participated in a battle between the towns of Lidel and Carwannock. His side suffered enormous losses, and an apparition in the sky blamed Lailoken for the deaths. This episode parallels Myrddinís experience at Arfderydd. Lailoken went insane and ran off to live in the forest like a wildman, where he spewed random prophecies of his own "triple" death and of the downfall of Britain. King Meldred hauled him into his court for amusement, but was decidedly unamused when Lailoken divined the adultery of Meldredís wife.
He was for a time at the court of Rhydderch Hael, revealed to King Meldred that his wife was adulterous and made several prophecies concerning his own death.
Lailoken was befriended by Saint Kentigern, who gave Lailoken his Last Rites at Lailokenís request, even though Lailokenís prophecy of his own death seemed impossible: he claimed he would die from a beating of sticks and stones, then from being impaled through the heart with a stake, and then from falling into water. Later, as Lailoken was wandering through a field near Dunmeller (Drumelzier), Meldredís shepherds spied him and stoned him. As he began to perish from the beating, he fell off a cliff into the River Tweed - and was impaled through the heart by a protruding stick.