Cloud Creature

The Cloud Creature was a type of gaseous monster found in DC Comics and an example of a "hidden species", a common trope in comic books that explored non-human species that inhabit remote regions of Earth rather than alien or magical invaders. A group of these cloud creatures would become the central antagonists of a 1960 comic story starring the popular speedster known as the Flash.

History
The cloud creatures were a malicious species of strange non-corporeal entities that lived under the Earth's surface and spent most of their time planning ways to invade the surface world. A group of such creatures was discovered by an unwary scientist in Lassen Volcanic National Park and soon they manifested on the surface via volcanic eruptions and began to attack via violent lightning and thunder storms.

This group of cloud creatures was eventually stopped by the Flash, who was able to exploit their one weakness: a strange nucleus that when struck, would turn them into harmless rain clouds (presumably by destroying the evil creature that had possessed them).

It seems likely that the cloud creatures either became extinct after these events or entered a state of dormancy as no further confrontation with the creatures occurred. Considering their hostile nature, it is unlikely they would stay hidden so long but it may also be possible any surviving cloud creatures have opted not to attack due to fear of similar reprisals.