From the Super Mario Wiki, the Mario encyclopedia
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Mermushrooms are a related species of Mushroom People that resemble merfolk. Although they have the heads of Mushroom People, the rest of their bodies appear to be entirely fish-like. The only place where Mermushrooms are known to live is their own kingdom, Aqua Land. The ruler of Aqua Land is King Neptune. Ironically, King Neptune himself does not appear to be a Mermushroom, and instead resembles a typical merman.
Mermushrooms are only ever seen in The Super Mario Bros. Super Show! episode "Mario of the Deep". Here, Barra-Koopa had taken over all of Aqua Land, and is forcing all the Mermushrooms to be his slaves. What their duties were as slaves is partially unclear, as the only duty they are shown to perform is gathering up the Gold Coins from the sunken ships in Aqua Land for Barra-Koopa.
One unnamed, female Mermushroom in particular is featured throughout the episode. In an attempt to seek help, she is seized by a Bloober, and is eventually rescued by Mario and Luigi. This Mermushroom then continues to guide Mario's group around Aqua Land, and explains the situation concerning Barra-Koopa's takeover to them. She eventually leads them to the dungeon where King Neptune is being held captive, so that Mario's group can rescue him, and stays with the group up until the end of the episode.