Mystery is a normal light green seahorse. She is bigger than an average seahorse, which are nearly Plankton's size. She has a swirl that 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 the two become friends, doing many things together.
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.
"Gary and Spot"
She appears in the dance party that Gary and Spot attend. However Marvin then catches them and captures them in the building. However, the duo later releases her along with the other animals, then realizing how alone Marvin is.
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."