'None that go by Carterhaugh
but they leave him a pledge
Either their mantles of green
or else their maidenheads

For if my love were an earthly knight
as he is an elfin grey
I'd not change my own true love
for any knight you have'

So what else would you call it?
OK Tam Lin isn't a real fairy.
The Queen of the Fairies transformed him from a human after he fell off his horse.
For seven years (isn't it always?)
And the night, very conveniently, was Halloween, so he's rescued before the Queen can turn him into a tree.
Hurrah.