As his name suggests, Seahorse is a seahorse. He has purple scales, darker purple-tipped fins, a curly tail, dark-blue mohawk-like spines, freckles, and black eyes. After his demoncism, he also wears glasses. As an employee at Reflectacorp Mobile, he wears a yellow shirt with a nametag.
Prior to his demoncism, Seahorse was a very aggressive, destructive, and bitter musician who played heavy metal. After his demoncism, he is turned into a well-mannered, upstanding (albeit robotic) contributor to society.
Seahorse first appears in the season 3 episode "Demoncism" as an employee at Reflectacorp Mobile, selling compact mirror phones to customers. Having undergone a demoncism to rid him of all his negative behavior, Star Butterfly and Pony Head ask him where Tom Lucitor's demoncism is being held. However, his drastic personality change make him a hindrance to Star and Pony Head's goals. When his behavior deeply upsets Pony Head, he manages to break through his personality changes enough to give her and Star a map to the site of Tom's demoncism.
In "Bam Ui Pati!", Seahorse helps Pony Head escape from Star and the Pony Head sisters so that she can watch her favorite Korean soap opera. He thwarts their pursuers by throwing boxes at them, distracting them with 3-D printed busts of Pony Head, and driving into an electrical storm. At the end of the episode, he gives Pony Head a new, artificial horn to replace the one she lost to Meteora Butterfly. Pony Head thanks him, but once again breaks up with him.
In "The Ponyhead Show!", Seahorse hacks the compact phones of everyone on Mewni in order to broadcast Pony Head's self-named talk show. In "Queen-Napped", he helps Pony Head and her sisters pull off an elaborate queen-abduction hoax in order to boost Eclipsa Butterfly's popularity.
In "The Right Way", Seahorse helps Pony Head and her friends protect Monster Castle from an invading Solarian Warrior, mostly serving as cameraman for "The Ponyhead Show!" He is later seen in "The Tavern at the End of the Multiverse", along with those wounded by the Warrior. He is briefly seen in "Cleaved", keeping vigil by the recovered Pony Head's bedside.
Pony Head is Seahorse's on-again/off-again girlfriend. Before he was demoncized, he wrote a song dedicated to Pony Head, and Pony Head considered him a romantic. After his demoncism, Pony Head hated his new personality and broke up with him. Since the events of "Demoncism", however, Seahorse has apparently made an effort to break through his personality change and make Pony Head happy again, despite how often they break up and get back together. In "The Monster and the Queen", Star mentions Pony Head has become interested in him again after he was imprisoned for Eclipsa's abduction.
- The Ponyhead Show!
- The Monster and the Queen (mentioned)
- The Right Way
- Here to Help
- The Tavern at the End of the Multiverse
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