One of the Black Cats.

Appearing at the first part of Attic, Basement and Beyond Nightmare are two black cats on ladders separated by a short distance that will throw mirrors up into the air and will shatter when they fall back down, harming Ned. He has to hit them three times each with his yo-yo and they will run up the ladders, then Ned can use the ladders. (The one on the left will warp you through a phonograph and the one on the right will take you into the School Nightmare.) When frightening both of the cats, a rocking sound can be heard. Ned can walk on the carpet to fall down to the Basement.

They are a representation of bad luck, being in addition to walking under ladders and breaking mirrors.