The Evil Manta.
This is an evil being whom the ancient Merpeople once locked in a volcano, but Ariel accidentally lets him loose. He spreads strife and hatred among the people of Atlantica before Ariel stops him. Later he tries to take over the seas several times, but is thwarted again and again by Ariel in The Little Mermaid.
In his first appearance, he tricked Ariel into freeing him from his volcano prison. He uses his sorcery to spread the shadow of prejudice. He is defeated when the sea creatures all band together and sing in harmony.
In his second appearance, everyone underwater believes that a boot is a weapon called a thingamajigger. It eventually comes into Manta's possession and he uses it to threaten Triton. He is defeated when Ariel shows up with a pair of boots and scares him away.
In his next appearance, Manta buys Triton's stolen trident from Mory the Eel. Ariel’s friend Urchin retrieves it but gets the Manta chasing after them. Manta kidnaps Flounder as a trade. Urchin shoots the trident, injuring both Manta and Flounder.
In his final appearance, Manta is training his son, Little Evil, to become a villain. Little Evil comes to be friends with Ariel, which infuriates Manta. Manta kidnaps Ariel and attempts to set a brain sponge on her. The sponge eventually turns on him but Ariel and Little Evil save him. He apparently reforms after that, if only a little bit.