Yaks entered Supernatural fandom on 19th July 2006 in an exchange between ignipes and deirdre_c. Yak fic (and associated icons) followed soon after.
Yak Fic means any story in which the characters either are attracted to yaks (possibly because of a spell), talk about yak fucking, are turned into yaks, or dress up in a yak costume. Generally comedic and obscene crack.
See the Crack page for a list of Yak fics.