The Dikironium Cloud Creature was an entity comprised of gases, feeding on red blood cells.
The creature was known to frequent Tycho IV, and, in 2257, it killed over 200 crew aboard the Starfleet starship USS Farragut, including commanding officer Captain Steve Garrovick. Then-Lieutenant James Kirk was amongst the survivors.
In 2268, now-Captain Kirk and his USS Enterprise crew met it again on Argus X and in space, where it killed several Enterprise crewmembers. Back at Tycho IV, Kirk and Garrovick's son, Ensign Garrovick, killed the creature on the planet's surface with an antimatter device. (TOS: "Obsession")