Rosario + Vampire

In the fanservice-heavy anime Rosario + Vampire, regular fella Tsukune Aono accidentally enrolls in a boarding school for monsters and supernatural creatures. Various monster girls take a liking to Tsukune, among them a mermaid named Tamao Ichinose.

Tamao, who has an unusual combination of green hair and orange tail, appears in the episode School Swimsuits and a Vampire. She promotes her swimming club in the hopes of recruiting members, but it's actually just a ruse to lure victims to be consumed by Tamao and her fellow mermaids. In the end it's Tsukune's lady friends who have to come to his rescue.