Find Ruby's Cat is a quest in Fallout: Brotherhood of Steel.
Before you head into the crater in Carbon, talk to Ruby and she will give you this side quest. It seems her cat, Mr. Pussy, has managed to get lost in the crater. The cat can be found, dead, in a house in the first set of buildings you come across in the crater. It's in a room in a building to the right, when coming from the crater entrance. Once your business is finished in the crater and you return to Carbon and find Ruby, you can return her the Dead cat, and she will tell you she has lost yet another thing, her necklace, starting another little side quest to follow your real duties.