|The following is based on Fallout: Brotherhood of Steel and is not canon.|
He went missing in the Carbon Crater, and subsequently Ruby asked the Initiate to find him. He is found dead in the crater (see map, right below), and the player character can bring his corpse back to her.
Interactions with the player character
Mr. Pussy is dead at the time he is found, so he appears in-game as the item "dead cat." Finding him is the goal of the quest Find Ruby's Cat.
Mr. Pussy appears only in Fallout: Brotherhood of Steel.