Nothin' But a Hound Dog: You may approach Old Lady Gibson about using Rey's brain in Rex. She will ask for 700 caps for this "favor" as he's like one of her family. She says he's getting old and he would probably need putting down soon anyway. You can kill him for free with a Barter check of 70. You can also snipe Rey from afar, then loot the brain off his body. After you kill him, you can use his brain for Rex. Using Rey's brain on Rex will increase Rex's ferocity in battle.