— SpongeBob SquarePants naming Mystery, "My Pretty Seahorse"
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.
In the Krusty Krab, she eats flowers, Krabby Patties, Old Man Jenkins, SpongeBob's spatula, the stove, SpongeBob's work hat, and Mr. Krabs' money. Mr. Krabs does not like Mystery and wants SpongeBob to let her go. SpongeBob 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.
"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.
Battle for Bikini Bottom - Rehydrated
SpongeBob appears riding her in a frames shot from "My Pretty Seahorse."
- Despite being female, one of her nicknames is "Sir Eats-A-Lot."
- Seahorses who look similar to Mystery are seen in the episodes "Whale of a Birthday" and "Breath of Fresh Squidward."
- SpongeBob regretted not naming Mystery "Debbie."
- Mystery appears on the water ride "Splash Bash" at Nickelodeon Land in Blackpool Pleasure Beach.