Flying Polyps.
The Flying Polyps are a species of 'half-polypous entities' that ruled the Earth before the arrival of the Yithians around a billion years ago. The Flying Polyps built black basalt structures in which they lived. They fought a war with the newly arrived Yithians and were defeated and forced into subterranean caverns and tunnels, but would eventually re-emerge and destroy the Yithian civilization in that era.
Flying Polyps were only partially material, and thus difficult to harm, but the Yithians found that electrical weapons were effective against them. They could generate powerful winds. Their presence was marked by five-toed footprints.
The Flying Polyps were amorphous beings described to be semi-polypous, and composed of matter only partly of this world. They often lapsed in and out of visibility, but could be recognised by the shrill whistling sounds which accompanied them. They had no eyes and required no light to navigate, as their other senses were powerful enough to pierce material obstructions. They had no wings, but were capable of flight, although when moving overland they left five-toed tracks of considerable size.
Conventional weaponary was largely ineffective in combating the Polypous Race, but they were vulnerable to certain kinds of electrical energy. Their intelligence as a race is debatable, but their minds were too strange to be understood by most other races.