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 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 have been eaten by Mystery. Mr. Krabs takes notice that SpongeBob never got rid of the seahorse as he was told to. SpongeBob 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.
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.