I just climbed into the blood pit, pressed <tab> and saw two black hounds following my group (moving independently).   Even after swapping out the black hound for the white wurm, the doppelganger hound continues to follow.. so now I've got two pets walking around, but only one in the slot.

 

I have no idea how this affects the gameplay, but it's definitely a bug (albeit an amusing one).

Since I can't find the edit key....

 

When I left the Endless Paths, I had three pets -- the wurm and two hounds.  Both hounds disappeared when I arrived at the Gilded Vale.  So, I'm assuming the bug is a non-destructive, non-garbage-collected pointer.

 

Unfortunately, although I do have savegames (immediately before and after the blood pit), the bug does not appear in either.