I'll give you three seconds to figure out what's wrong with American AIBO. The first two don't count.
On topic:
http://www.fanfiction.net/read.php?storyid=132177
Predator vs. Buffy. It comes close to jumping the shark at the end of the second and third chapters, but it's a very entertaining read throughout. Plus it's got Evil Spike, and Evil Spike was sooooo much cooler.