As we learn in the BtvsSeasonOne episode ThePack, animal spirits are not the same as human souls, and yet they are. Just as apples and oranges are both fruit, animal spirits and human souls are, uhm.. er, ethereal entities but they operate in dramatically different ways, and animal spirits lack rational conscience. Animal spirits are also not the same thing as demonic ethereal entities. Just as there's a difference between XanderHarris' human soul and the hyena entity which possesses him in ThePack, so too is there a difference between {{Angel}}'s human soul and {{Angelus}} vampiric demon uh, thingy.
Then in the BtvsSeasonFour Thanksgiving episode, we learn that human souls which worshipped animal spirits in their lives may sometimes have above ordinary powers in their afterlife.