When Santa's Little Helper falls ill and needs an expensive operation, the whole family makes budget cuts.  The family ends up resenting a saved Helper, causing him to run away.  Helper eventually becomes a guard dog for Mr. Burns and just as he's about to attack Bart, he remembers fond memories, and comes back to the family.

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The Simpsons Season 3 Episode 19 Quotes

Veterinarian: This is the part of the job I hate the most. (Tosses hampster into trashcan with mini-basketball hoop)
Homer: Hey, you did the best you could.
Veterinarian: I love animals. I spend my life saving them and they can't thank me. Well, the parrots can. Let's see what's wrong with this one.

Homer: (about Santa's Little Helper) Aw, how come he gets meat and we don't?
Marge: You wouldn't want what he's eating, it's mostly just snouts and entrails.
Homer: Mmmm, snouts.