wrath rats don't mate! they are obviously the magical construct of some lunatic alchemist.
can there be lunatics in yliakum if there's no moon?
Just because they were initially constructed, doesn't mean they weren't given the ability to operate and thus mate, on their own. Otherwise you'd need constant production of them, and since I'm not sure there are any guilds devote to necro-surgery, I'm assuming they're finding a way to procreate.