Dead cat is a quest item in Fallout: Brotherhood of Steel. It is actually Mr. Pussy, a cat once owned by Ruby the prostitute.
The dead cat is your standard, run-of-the-mill expired cat. Its owner, Ruby, called it "Mr. Pussy". This item is a part of the quest, Find Ruby's Cat.
The cat can be found, dead, in a house in the first set of buildings encountered in the Carbon Crater. It's in a room in a building to the right, when coming from the crater entrance.
Mr. Pussy is located at the nr. 4 marker