Gryff, the animal headed people are actually called Manitoba in a series of books by Charles DeLint. See Moonheart. In this series they are actually very Shamanically oriented, and I've always thought very cool. This series of books about a mythical town in Canada is perhaps one of my most favorite sets of books. So he is sort of using the region of Manitoba as a spirit name for his creatures.
I think they are spirit helpers, the spirits of animals come to us with some features of humans so as to be more acceptable to us humanfolk. I don't know the actual true mythology here, and perhaps this is a Mi-Shell question. What I do know is that this a theme in several more pagan novels I've read. So there has to be an indigenous source to the appearance of the animal headed people. Perhaps it is also a Celtic invention since it is used here in the Greenwood/Wildwood.
I believe that the Shamans in Celtic times would wear the animal head of certain animals to honor and call down the spirit of that animal. In the Clan of the Cave Bear books they did this with the Bear, adorning the Shaman of the Cave Bear with a skin of the bear and the head intact.
So in the Wildwood I think this is supposed to be the Spiritual leader either wearing the animal, or the animal spirit coming to the tribe as part animal, part human. Hence, we are brothers and sisters, animals and people, and should be seen as such.
Further, there is a dangerous quality to these spirits, sort of a warrior like attitude, or gaurdian of something/ some one.
ETA:
For clarification purposes:
I do not think that the wearing of an animal makes someone a shaman.
