Hippocampus, more commonly known as Seahorse, is the genus of Mystery.
The green version of a seahorse appears in the episode "My Pretty Seahorse." Her role in the episode is as SpongeBob's temporary pet after she develops an interest in the flowers SpongeBob plants in celebration of spring beginning.
After Scooter mistakes Mystery for a brand new kiddie ride that Mr. Krabs puts outside of the Krusty Krab and gets harmed by her for inserting a coin inside her "coin slot," Krabs goes outside to see what is causing trouble and encounters what he thinks is a monster, only for it to turn out to be SpongeBob's new adopted seahorse. However, Mr. Krabs forces him to get rid of her or else he will be fired. Heartbroken, SpongeBob agrees to do so, though later hides Mystery in a cabinet in the kitchen to prevent Krabs from seeing her.
After coming out and eating a customer's order, SpongeBob's spatula and Krusty Krab Employee Hat, part of the grill, and even Old Man Jenkins, Mr. Krabs takes notice that SpongeBob never got rid of the horse as he was told to. As a result, he is forced to actually do so this time, but is far too attached to take action. However, after Krabs shows sympathy and tells a childhood story involving his love for a nickel in contrast to SpongeBob's attachment to Mystery, SpongeBob agrees to let the horse go.
However, after hearing news from Squidward that Mystery ate a majority of the money in Mr. Krabs' cash vault, he and SpongeBob chase after her, but fail to track her down.
The gray version of seahorse appears in the episode "Nasty Patty." It does not play any special role; he or she only appears as a photo on a horseradish container.
Seahorses will eat almost anything.
Mystery is shown eating anemones, an anemone bookmark, Krabby Patties, Old Man Jenkins (she spit him out eventually), part of the grill, part of SpongeBob's spatula, Mr. Krabs' money, and SpongeBob's Krusty Krab hat.
- The animal is mentioned in the episode "Bubble Buddy."
- A seahorse is put in "Opposite Day" as part of the pin-the-tail-on-the-seahorse party activity.