Eris (Barbie: A Mermaid Tale)

Eris is the main antagonist of the movie Barbie:A Mermaid Tale, She's the sister of Queen Calissa and aunt of Princess Merliah, she was portrayed by Katleen Barr

Story
Eris is the sister of Queen Calissa. When Calissa and Eris were young, it was time for the Changing of the Tides Ceremony and only one could take the throne. Princess Calissa was more kind and fair than her sister, plus she could spin Merilia, whereas Eris couldn't, so she took the throne. Eris started to hate her sister.

After years, Eris overthrew Calissa and took the throne. She ruled Oceana for 15 years selfishly. The ocean became sick since the Merillia was unhealthy, as Calissa was unhappy locked up. But when Queen Calissa's half-human, half-mermaid daughter (Merliah) came to Oceana, Eris was deposed.

She later escaped from her prison with the unwitting help of Kylie Morgan. Eris revealed she had the power to make anyone live through their worst nightmare, and with it, she planned to take the thrown of Oceana and spin merillia. Ultimately, she was defeated and experienced her worst nightmare, having legs instead of a tail.