Tam Lin and Thomas the Rhymer are two different stories about men who find their way (through one method or another) into the clutches of the Queen of the Fairies. Thomas the Rhymer stays for seven years, and when he finally leaves, he is blessed/cursed with true speech, the inability to say anything but the truth. (He is also called True Thomas because of this.)
Tam Lin manages to send a message to a girl named Janet, to meet him at such-and-such a place, where all the Faerie will be travelling. He asks her to pull him down from his horse, and to keep a hold of him, no matter what happens, until he turns into a man again.
Janet does so, and Tam Lin turns into all manner of horrible animals, until finally he turns back into himself, and the Queen is defeated, for the time being.