So what does he want to be called? Just a rat? But - there are actual rats, which are already called rats. Unless those don't exist in this universe, "ratman" seems like a pretty reasonable term. He has both rat and humanoid features.
Serious answer? He'd be called a Rat-Man in the language of humans, and an original word in his own language
Yeah. Him calling the man "man-rat" makes no sense though. For it to make sense, there would have to be something even more human and more distant from rats, so that he could be viewed as half rat.
The rat...person would call themselves a word in their language that would translate to man language as "hu-rat". the man would thus be a man-rat.
But the human calls the rat-person a ratman because they seem to be something between a human and a rat. The human would call a regular rat a rat. Why would the rat-person call the human a man-rat, if the human is a regular human? Who would he call man then?
It's a false equivalency on the Ratman's part. There exist rats. There exist men. He's a cross somewhere between, where the man is just a man. I mean, provided there isn't some kind of miniature, unintelligent human analog in this world.
Link to source, for fellow sauce enjoyers
https://www.onepagerules.com/comic-episodes/squip-season-2-episode-4
The daemon is not his foot (badly drawn, so it looks like that?). They are "buddies" and travel together.
We could just rename humans to slutfolk. They'll bed anything, which is why there's half elves, half orcs, dragonkin, ...