This isn't a specific creature, but I think it would be kind of cool to be able to hire neutral troops from different camps or tribes during the campaign mode. For example, a tribe of werewolves might devote some soldiers to you if you slay a certain amount of vampires from a different tribe.
This can also be applicable to other "cultures." Since this is a major battle between good and evil, Eastern warriors should be available for hire. Samurai would be devoted to good, but ninjas and maybe special Chinese crossbowmen (chu-ko-nus for those AoE fans) should be able to be hired for both heroes and keepers.
I know that creatures themselves are supposed to be mercenaries, but I was just suggesting there could be a different way to get creatures to fight for you other than torture.