Mystery is a seahorse that SpongeBob adopted and let go in the episode My Pretty Seahorse.
Mystery is mainly a normal green seahorse. She is also bigger than any average seahorse, which are nearly Plankton's size.
For a while though, SpongeBob considered naming her Grace, or Majesty, or maybe even Debbie, but finally she named it Mystery. Her name matches because if she eats something and walks away, then nobody knows and it's a mystery problem. However, in the KK, she ate flowers, Krabby Patties, Old Man Jenkins, SpongeBob's spatula, all the Krabby Patties, the stove, SpongeBob's hat, SpongeBob's book chapter mark, and Mr. Krabs' money. Mr. Krabs doesn't like Mystery that much, and wants him to let go Mystery, although he lies about Mystery having a seizure. SpongeBob was afraid for Mystery to go, but Mr. Krabs told him when he spent his first dollar, and he finally agreed with him. Mystery was let go by SpongeBob, who said his last words to Mystery, and he began crying while Mr. Krabs tells him that he did the right thing. Soon, Squidward had noticed it because he saw that she ate the bank, and SpongeBob & Mr. Krabs were chasing her. She was never seen or heard of again after the episode. It was an mystery story of the relationship between SpongeBob and the Seahorse.