Avalanche? I've never heard of that move. Let me do a little research. I've got Rhyhorn's page pulled up, and he can't learn the move Avalanche by any means. Are you sure you aren't confusing it with something else? No, you're not. Rhydon and Rhyperior can be taught it, but not Rhyhorn for some reason. Buster will simply have to wait a few levels before he learns it.