Medusan

The Medusans are a highly-intelligent, telepathic race from the Alpha/Beta Quadrant region. They are one of the few noncorporeal species to be members of the Federation.

Medusans are known throughout the Federation as mathematical and navigational experts without equal. Their thoughts are the most sublime in the galaxy, however, their physical appearance so utterly hideous that the sight of a Medusan would render a human mad. Vulcans are capable of viewing Medusans, but only with the use of a specially filtered visor. For these reasons, Medusans usually use an opaque carrier pod when interacting with other species. (TOS: "Is There in Truth No Beauty?")

In the mid-24th century, specialized habitats were incorporated into several Starfleet vessels so that Medusans could serve as onboard navigators through the use of the Interactive Navigational Control System. This would allow the Medusans to operate in symbiosis with the ship's computer in case normal navigational systems were offline. (FASA: Star Trek: The Next Generation Officer's Manual)

Known Medusans

 * Kollos - Federation ambassador during the mid-23rd century.