I recalled reading somewhere in the core rules that animals won't approach undead or aberrations, but now I can't find any such passage. The PHB does state that getting animals to attack "unnatural" creatures does require special training, so perhaps I just extrapolated from that. Or perhaps I'm remembering something I read in the 3e core rules that wasn't carried over to 3.5. I thought I'd check and see what other folks know on the subject.