The Jet-Propelled Monsters were an alien species that inhabited a certain planet in another galaxy. They prey on inhabitants of other worlds, destroying people and cities.
In one Red Kryptonite-induced delusion, Supergirl killed a large number of them using Fatal Vision, as a way to atone for all the people she had accidentally killed that way in the delusion. ("The Strange Bodies of Supergirl!")
- As these creatures only appear in Supergirl's delusion, it's possible they don't exist at all. However, her reference to having learned of them some time ago seems to show that they do.