Octopus King is a one-time villain from the Spongebob Squarepants comics. Octopus King is an evil octopus king, who used to be arch-enemies with Mermaid Man and Barnacle Boy back in the day but has since then retired. Despite his retirement, Mermaid Man and Barnacle Boy still view him as a threat and refuse to let their guards down around him. He appeared in the comic book issue, "Mermaid Man and Barnacle Boy vs. The Octopus King".
In "Mermaid Man & Barnacle Boy vs. The Octopus King", Spongebob asked Mermaid Man and Barnacle Boy to do something heroic. Not wanting to let their fan down, the duo pondered what they could do to behave heroically. They thought of fighting a supervillain and wondered if any of their former rivals were still alive. Barnacle Boy remembered The Octopus King and the two set out to stop him.
The Octopus King was currently retired at this point and had no intention of doing anything evil anymore. However, Mermaid Man and Barnacle Boy still saw him as a threat for some reason and challenged him to a battle. Octopus King was spending his retirement, sleeping. He wasn't even aware of Mermaid Man and Barnacle Boy so he didn't do anything.
Mermaid Man and Barnacle Boy yelled at Octopus King and started attacking him, despite the latter doing nothing. In their fight, they got tangled up in his tentacles and stuck, without Octopus King even intending to hurt them, as he was still asleep the entire time. At this point, Mermaid Man and Barnacle Boy realized that Spongebob was no longer paying attention to them and was now focused on a bug he saw on the ground, meaning that the two of them had just gotten themselves tangled up in the Octopus King's tentacles for no reason.
Despite his inaction in his central comic book issue, Octopus King was shown being awake and wreaking havoc on the posters for the comic book issue, "Spongebob Comics Annual Super-Giant Swimtacular No. 1", where he, along with every other Mermaid Man and Barnacle Boy villain swarmed to attack The Aquatic Adventurers.
Octopus King would later have a cameo on the cover of the 2018 issue of "Spongebob Freestyle Funnies", where he and the other Mermaid Man and Barnacle Boy villains surrounded Spongebob and prepared to attack him.