Pet Rocks are rocks that Patrick, Squidward, and SpongeBob adopted or treat as pets.
Patrick's Pet Rocks
Googly Rocks
Patrick's Googly Pet Rock
Patrick's Googly Pet Rock is seen in the episode The Googly Artiste. Patrick made it with a lot of messy, googly eyes and glue, and sold them as art.
Rocky
Rocky
Rocky is a rock which Patrick mistook for a snail. Patrick entered Rocky in a snail race competing against SpongeBob's pet snail Gary and Squidward's pet snail Snellie. This rock actually won the Snail Race in the episode The Great Snail Race. Rocky appears to be the only pet rock that has "life."
Pete
Pete
Pete is a rock with googly eyes and a drawn mouth that was given to SpongeBob by Patrick. SpongeBob was going to throw it away until Patrick taught him about 'sentimental value' in the episode Sentimental Sponge.
Squidward's Pet Rocks
Squidward's Pet Rock Collection
In the episode, Once Bitten, it is revealed that Squidward has a pet rock collection, which was slimed by Gary in the same episode. They were rocks that have eyes.
SpongeBob's Pet Rocks
SpongeBob's Pet Rock
In the episode, The Googly Artiste SpongeBob creates a googly-eyed pet rock for Patrick to model after him.