The Colloquy of Colum Cille and the Youth at Carn Eolairg.
Some say he was Mongan the son of Fiachna. Said Colum Cille to him: ‘Whence hast thou come, O youth?’ said Colum Cille.
Respondit iuvenis: ‘I have come’, said the youth, ‘from unknown lands, from known lands, that I may know from thee the spot in which knowledge and ignorance have died, and the spot where they were born, and the spot in which they are buried’.
Respondit Comm Cille: ‘A question’, said Colum Cille. ‘What was this lake which we see, formerly?’
Respondit iuvenis: ‘That I know. It was yellow, it was flowery, it was green, it was hilly, it was full of drink, it was ..., it was rich, in silver, it was full of chariots. I abandoned it (?) when I was a deer before deer, when I was a salmon, and when I was a seal of great strength, when I was a roving wolf, when I was a man, I took up my abode (?) with sails, a yellow sail, it carried a green sail, it drowned a red sail under ... of blood. Women shouted to me. Though I know neither father (nor) mother ... I speak (?) to living men ... to the dead’.
Said Colum Cille again to this youth: ‘... islands to the west of us, what is underneath them?’
The youth answered: ‘There are underneath them tuneful longhaired men, there are terrible pregnant kine underneath them whose lowing is musical, there are herds of deer, there are horsely horses, there are double-heads, there are triple-heads, in Europe, in Asia, in unknown lands, a green land above its many borders (?) to its estuary (?)
‘Enough so far’, said Colum Cille. Colum Cille went aside with him to converse with him and to ask him about the heavenly and earthly mysteries. While they were conversing for half a day or from one hour to the same hour on the next day, Colum Cille’s monks were looking at them from afar.
When the conversation had come to an end, they suddeiily beheld the youth vanishing from them. It is not known whither he went. When his monks asked Comm Cille to make known to them something of the conversation, Colum Cille said to them that he could not tell them even one word of all that he had spoken to him, and he said it was a proper thing for men not to be told.
Meyer, Kuno. "The Colloquy of Colum Cille and the Youth at Carn Eolairg." Zeitschrift für Celtische Philology. vol. 3. 1899.
It's intersting to compare this short tale with the story of Taliesin, another divine youth of shapeshifting properties.