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.
For a while, SpongeBob considered naming her Grace, Majesty, or maybe even Debbie, but finally decided upon the name Mystery. Her name matches because if she eats something and walks away, then nobody knows and it's a mystery problem. However, in the Krusty Krab, she eats flowers, Krabby Patties, Old Man Jenkins, SpongeBob's spatula, the stove, SpongeBob's work hat, SpongeBob's floral bookmark, and Mr. Krabs' money.
Mr. Krabs doesn't like Mystery, and wants SpongeBob to let her go, although SpongeBob lies about Mystery having a stomachache. He 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's for his own good, and finally agrees with him.
Mystery is then let go by SpongeBob, albeit reluctantly. Soon, Squidward notices that she ate the bank, and SpongeBob and Mr. Krabs start chasing after her, yelling at her to come back.
Purple seahorses appear in "Neptune's Spatula." They are summoned by King Neptune and bring him an apron to wear. An emblem of a purple seahorse can be seen on Neptune's belt buckle in every episode that physically features him.
Mystery is shown eating anemones, an anemone bookmark, Krabby Patties, Old Man Jenkins (she spits him out eventually), part of the grill, part of SpongeBob's spatula, Mr. Krabs' money, and SpongeBob's Krusty Krab hat.