I thought the whole point of the wolfbrothers was, their soul was, for lack of a better word, welded to a wolves, which allowed them to access both. I mean, Darkhounds were Shadow-twisted souls of wolves, we know this because bodies of wolves were never dragged off or used.

As to whether there's "deerbrothers" or "eyelash-mitebrothers" is another question entirely, but I feel there's enough support for wolves having immortal souls in WoT.