Mystery is mainly a normal light green seahorse. She is also bigger than any average seahorse, which are nearly Plankton's size.
For a while, SpongeBob considered naming her Grace, or Majesty, or maybe even Debbie, but finally he 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 Krusty Krab, she ate flowers, Krabby Patties, Old Man Jenkins, SpongeBob's spatula, all the Krabby Patties, the stove, SpongeBob's hat, SpongeBob's floral bookmark, and Mr. Krabs' money. Mr. Krabs doesn't like Mystery that much, and wants him to let Mystery go, although SpongeBob lies about Mystery having a stomachache. 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 that she ate the bank, and SpongeBob and Mr. Krabs started chasing her. She was never seen or heard of again after the episode. It was a mystery story of the relationship between SpongeBob and the Seahorse.