Technically speaking monster girls would be a sub-species of humans as a whole cause in order for something to be recognized as a species it has to be genetically different enough to where crossbreeding with other species is either impossible or really hard.

The reason why we classify it this way is that way is funnily enough has to do with humans. Originally there were dozens of hominids but because our ancestors interbred with them so fast that gave rise to modern humans.

This is a nice image. But… ELVES ARE NOT MONSTER GIRLS.

Freaking plebs.