Mellitus

From Memory Alpha, the free Star Trek reference

The mellitus is an animal from Alpha Majoris I colloquially referred to as a "cloud creature". In its normal state, as it moves, it exists as a gas held together by a highly cohesive electromagnetic field. When the mellitus stops, however, it becomes solid.

Though real, the mellitus is 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")