While fighting with a necromancer around, I noticed that when a monster gets resurrected, it sometimes upgrades.
http://imgur.com/QcZDjOu <-- the image includes the log which shows me killing a feral hunter, then the necromancer resurrecting a feral predator from the corpse. All other dead bodies were butchered before I lured in this pack of zombies, so it could not have been a corpse of an existing predator. In a previous pull, there was a zombie that became a zombie master.
It isn’t every resurrection, as zombie soldiers don’t seem to upgrade. I tested this by killing it and letting the necromancer resurrect it over and over.