Mr. Krabs' pet scallop is a character who only appears in the episode "Model Sponge." He goes back to the wild after Mr. Krabs had raised him.
SpongeBob is about to clean the dining room floor when he passed Mr. Krabs in his office, on the phone with an unknown character and referred to his pet scallop as "little guy." At first, SpongeBob thinks he is talking about him because it sounded like Mr. Krabs was bragging about him. As the conversation goes on, SpongeBob hears Mr. Krabs say, "...I'll have to let the little guy go today." SpongeBob thinks that Mr. Krabs is going to have to fire either him or Squidward. After a quick check with the latter, it becomes clear to SpongeBob that he was the "little guy."
After SpongeBob leaves the Krusty Krab in desperation, the episode goes back into Mr. Krabs' office, where he is still talking on the phone. It is at this time that it was revealed he was talking about his pet scallop, which was getting too big for his cage. He then hangs up the receiver and lets the scallop free in the wild.