Caligo

Caligo is the main antagonist in Barbie: The Pearl Princess. He is the queen's brother and wants to make his son Fergis become king, but can't because Princess Lumina was born. He asked the old mermaid named Scylla to take her far away so he can get his son on the throne.

Trivia

 * Caligo is the only merperson to have a shark-like tail.