I'm afraid I'm a bit light on the details. It's a fairly recent one, though, posted sometime in the past few months. It concerns hyena!Xander, wherein the hyena fully integrates with him (a phrase used in the fic). For most of the fic, he and Spike keep arguing/fighting over who is the more alpha/dominate one, which pisses off Angel who is in Sunnydale for some reason. Xander actually comes out on top (literally and figuratively) in that argument, near the end. I wish I could remember more of the plot, such as why Angel is there or what the Big Bad of the Week is, but that's about it.