I'm looking for a fic where Xander has a cat tail since he was little because when he was at the museum with Cordy and Willow they went to see a wishing stone and Willow wished Xander had a cat tail and Cordy wished the tail was a lively one to entertain Willow. But the thing is that Willow denied it all and never wanted to see the tail. I think Spike finds out and the tail likes him (but I'm not sure)
Or it's by Voracity called Furry Investigations
On the BUffy Single stories page.
and thank you for the help!