Ok, did a search of the forums and didn't see this one anywhere, so I'm guessing it hasn't been reported.
Just started a fort in a Haunted Tropical Grassland w\ stream in the hopes of spicing things up (and given the Zombie Giant Jaguar lurking out there it seems I've done just that.)
Having walled myself and my wagon up until I can get my two knights better equipped, I wasn't paying allot of attention to what was happening outside. Curious to see what was wandering around up above I checked the units screen and was amazed to find a massive list of 'deceased' zombies and skeletons. I looked over by the river and sure enough there's a massive slaughter zone of zombie warthogs, skeletal fish, a herd of zombie gazelles... and then I saw what had caused it; two zombies were still standing, a warthog and a tigerfish. and then they attacked each other. It seems the roaming undead, classed as wildlife, are hostile with the map feature undead, classed as undead.
Not that I'm ungrateful to those zombie hogs for clearing up the problem of piping that stream over to my fort, but that doesn't seem like the way they're supposed to behave. I knew the dead hate the living but apparrently there is also much enmity between the walking dead and the... uh, swimming\flopping dead.