The Medusans were a highly intelligent telepathic and empathic 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 to humanoids that the sight of a Medusan rendered any viewer mad, and soon afterwards 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 the Gorgon "Medusa" whose gaze had the power to turn to stone anyone who looked into her eyes.
According to the story outline and script of the episode, their name was spelled "Medeusan".
A planet was seen on screen in the original "Is There in Truth No Beauty?", but the remastered version of the episode, a Medusan vessel was seen instead.
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 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.