The Sinister Felines From Atop the Litterbox are the pet cats of the Delightful Children From Down The Lane and the enemies of the Hamsters Next Door. Both are animal counterparts to their owners.
They only appear in Operation: H.A.M.S.T.E.R., in which they crash the party the Hamsters are holding and capture them. The Hamsters Next Door then battle the cats throughout the Sector V Treehouse and ultimately defeat them, in a Tom and Jerry-esque fashion.