In the manga +Anima (2001-2005), what is meant by the titular term is a human who possesses the secondary traits of a particular animal, like the wings of a bird or the strength of a bear. That sounds like a great premise for a superhero fantasy, but this series chooses to spin it a bit differently. Unfortunately for the +Anima, being so "different" isn't necessarily a blessing within a prejudiced society.
The series's four main characters are all +Anima with different animal bases. For example, Husky is a fish +Anima. Part-human, part-fish. So basically, a mermaid. ...Except, in this case, not a "maid."