The Melusine (sing. Melusine, plural undetermined) are a race of creatures in Fontaine.
Melusine were first shown in the Version 3.8 Special Program, as part of a story narrated by Alice showcasing a teaser of the nation of Fontaine.

Overview
A Melusine is described to be diligent with its duties, having a great sense of justice. The Melusine shown in Alice's story sports a garb similar to a law enforcer, which could be one of, if not the supposed vocations Melusine partake in.

Appearance
The Melusine shown in Alice's story is bipedal, has a short stature, fawn-like in appearance, has little wing-like appendages on her back and a long, tufted tail. A blurred figure of what seems to be another similar Melusine is seen aboard a boat.
It is currently not known how much variance appearance-wise is present among Melusine.

Source: https://genshin-impact.fandom.com/wiki/Melusine