Mystery is mainly a normal green seahorse. She is also bigger than any average seahorse, which are nearly Plankton's size.
Mr. Krabs seemed not to like Mystery after SpongeBob lied to him about keeping outside but catching him when Mystery had a seizure. Her name matches because if she eats something and walks away, then nobody knows and it's a mystery problem. For a while though, SpongeBob considered naming her Grace, or Majesty, or maybe even Debbie. 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. Soon, Squidward had noticed it, and SpongeBob & Mr. Krabs were chasing her, and it is a mystery where Mystery is. It is unknown if they ever got her or where she went. Her sounds were provided by Frank Welker.
- Seahorses similar in appearance to Mystery are seen in the episode Whale of a Birthday.
- SpongeBob was going to call her Debby.