I vaguely remember finding someone a few years back who claimed to have drawn their trans OC as a mermaid because of the complete absence of human genitalia, and this being informed by her (the character’s, but really the artist’s) discomfort with that area.
I also remember hearing on a non-binary podcast (it’s deleted now, but the hosts were called Hyacinth and Reed) that trans people on general tend to associate themselves with mythical beings more broadly. And if I am to speculate beyond this, it’s that they often tend to see themselves as inhuman, either because they’ve been othered by the rest of society (what they’ll say), or because they want to be different and special from what is merely human (what they’ll think but not admit).