In the fic Xander gets turned into a deer-type demon. I think he might have had antlers and hooves, but I'm not really sure. I know he had the ability to run really fast, faster than a vampire even. At one point he uses this ability to run away. I'm pretty sure Angel and Doyle were involved. I remember this being a longish fic too, if that helps anybody.
Thanks you guys, hope you can help me!