Hellcat

Hellcats can only be obtained after starting the Evil Dave section of the Recipe for Disaster quest. To turn your cat into a Hellcat, head to the basement in the house of Evil Dave's Mother. Tell your kitten or cat to "Chase Vermin". After it has killed a sufficient amount of hellrats, your cat will become a Hellcat.

Hellcats are exactly the same as regular cats except when you talk to them while wearing a catspeak amulet. To turn your Hellcat back into a regular cat, just feed it some milk.