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 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. (TOS: "Wolf in the Fold")