Kaiju
Literally "strange beast." Encompasses a variety of (often quite large) monsters.
The most common type of kaiju in the Sol system might be classified, scientifically, as "gigafauna;" creatures like Red King, for instance, are simply large animals that sometimes come into conflict with humans over basic needs like food, shelter, and space. While uncommon during the Common Era, these creatures have become increasingly common since the 0040s.
Other kaiju come from outer space. Though not always malicious, a kaiju that travels to Earth from outer space has a much greater likelihood of being hostile or wanting to invade the planet. An Angel, strictly speaking, falls under this definition of kaiju.
Finally, some kaiju seem to be associated with areas where negative feelings are common and go unexpressed. Despite these origins, these types of kaiju can be benevolent or malevolent.
Any type of kaiju can threaten humanity.
The line between "kaiju" and "alien" is fairly thin. Much like with "mobile armor," scientists throw the term at each other primarily to test whether everyone else in the room agrees on the same definition.