Mystery is a normal light green seahorse. She is bigger than an average seahorse, which are nearly Plankton's size. She has a swirl which contains a tail.
She has similar mannerisms to a regular horse, eating a lot and neighing.
Role in series
"My Pretty Seahorse"
When SpongeBob first finds her, he wants to have her as a pet. He considers naming her Grace, Majesty, or maybe even Debbie, but finally decides upon the name Mystery. SpongeBob tames her and then become friends, and they do many things together.
Mr. Krabs does not like Mystery and wants SpongeBob to let her go. The latter is afraid for Mystery to go, but Mr. Krabs tells him a story about when he spent his first dollar to convince him that it is for his own good, and SpongeBob finally agrees with him.
Mystery is then let go by SpongeBob, albeit reluctantly. Soon, Squidward notices that she ate Mr. Krabs' money, and SpongeBob and Mr. Krabs start chasing after her in an attempt to make her come back.
What's Your Bikini Bottom Pet?
She is one of the pets featured on the title screen in this online game.
Despite being female, one of her nicknames is "Sir Eats-A-Lot."