Monster girls are a special type of humanization, where the anthropomorphic character is a mix between a creature (both real and fictional) and a girl. Monster girls can also refer to moe versions of already existing monsters, such as vampires or trolls.
Do not use this tag for full-fledged monsters who switch between completely human and completely monstrous forms (due to shapeshifting or lycanthropy for example), only for characters who display both human and monster traits at the same time ("werewolf" who has wolf ears and tail, but otherwise human form).