Mellitus

The mellitus was a lifeform from Alpha Majoris I colloquially referred to as a "cloud creature". In its normal state, as it moved, it existed as a gas held together by a highly cohesive electromagnetic field. When the mellitus stopped, however, it became solid.

In 2267, the USS Enterprise (NCC-1701) computer considered it similar to the Redjac entity. Hengist dismissed the lifeform as being nothing more than something out of a fairy tale, but Captain James Kirk recalled that he had seen the Mellitus. 

Mellitus