The Simpsons Episode 3.19 Dog of Death

Santa's Little Helper is rushed to the hospital to undergo an emergency operation. Homer is saddened to tell Bart and Lisa that they just can't afford the $750 for the operation, but seeing how much everyone--including himself--loves the dog, he resolves to find a way to pay for it. To save up the money, everyone must give up their small luxuries and begin to resent the dog for forcing them to lose out on the things they enjoy. Hurt and dejected, Santa's Little Helper runs away from home, only to be adopted by Mr. Burns, who trains him to become one of his vicious attack hounds. After a long brainwashing process, Santa's Little Helper becomes a bloodthirsty killer. When Bart goes to Burns' mansion to retrieve his dog, Santa's Little Helper tries to attack him but remembers all the good times they had and snaps out of his brainwashed state. The dog then returns to the family that loves him.