In Melanesia there is a belief in the tamaniu or atai which is an animal counterpart to a person. It can be an eel, a shark, a lizard, or some other creature. This creature is corporeal, can understand human speech, and shares the same soul as its master, leading to legends which have many characteristics typical of shapeshifter tales, such as any death or injury affecting both forms at once.I'm looking into the source. Could be an interesting one.
On a related note, I've been reading my Jung, lately. I may pop back in to stick up some quotes from that.
Note to self:
Hamel, F. (1969). Human Animals, Werewolves & Other Transformations. New Hyde Park, NY: University Books. p. 21. ISBN 0-8216-0092-3.