The Medusans were a telepathic non-corporeal race. Their thoughts were the most sublime in the galaxy, but their physical appearance was exactly the opposite. Their appearance was so utterly hideous that the sight of a Medusan rendered any Human mad, and soon afterward caused death by massive organ failure. Vulcans were capable of viewing Medusans, but only with the use of a specially filtered visor. For these reasons, Medusans usually used an opaque carrier pod when interacting with other species. The Medusans were renowned for their navigational abilities. (TOS: "Is There in Truth No Beauty?")
List of Medusans
The Medusans were named for the Greek mythological character of Medusa whose appearance was so hideous that anyone who laid eyes on her was turned to stone.
According to the story outline and script of the episode, their name was spelled "Medeusan", while the non-canon Star Trek: The Next Generation Officer's Manual spells it "Medusan". The Officer's Manual also states that Medusans were permitted to serve in Starfleet as emergency navigators on board Galaxy-class starships. Such Medusans were honorary lieutenant commanders and had special quarters off of an emergency navigation center, which kept them isolated from the rest of the crew.
A Medusan character appears in the A Time To... novel series.