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

Smithers: People like dogs, Mr. Burns.
Burns: Nonsense, dogs are idiots. Think about it Smithers, if I came into your room and started sniffing at your crotch and slobbering all over your face, what would you say?
Smithers: Umm...if you did it, sir?

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.