I think the images pretty much speak for themselves. LOL. Though, I will note that I notice there to be an extreme lack of cats present in these Halloween pet photos. Quite simply, cats will not stand for this kind of nonsense! :P
Anyway, here's the answer to why pets hate Halloween...
Why Pets Hate Halloween - Part I
For part II of Why Pets Hate Halloween, go here. Lots more photos of pets dressed up in Halloween costumes. Poor little fellas.