I thought for sure it was, but can't see any sign of it on LJ or AO3. I swear I'm remembering this right, that it was...oh wait, it was Willow/Dawn, yeah, but it was giant!Dawn. (For some odd reason, this fic is not on AO3 :)

Although I do swear I read a centaur!Dawn at some point, having found the above makes it harder to pinpoint.