The mellitus was an animal 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.

Though real, the mellitus was sometime dismissed as being nothing more than something out of a fairy tale. The USS Enterprise computer considered it similar to the Redjac entity. (TOS: "Wolf in the Fold")