The Melkotian buoy was an orbital warning buoy placed by the Melkot, a telepathic and xenophobic race, to warn others not to encroach on their territory in space.
The Federation starship USS Enterprise made first contact with the Melkot through an encounter with the Melkotian buoy in 2268. However, Captain James T. Kirk ignored the warning and ordered the vessel onward. The Melkot then attempted to punish Kirk and members of his crew by trapping them in a bizarre recreation of the events leading to the gunfight at OK Corral, which were intended to lead to their deaths.
The Melkot released Kirk and his landing party from their illusion following Kirk's display of mercy. They caused their buoy to emit unstable M-rays at energy levels beyond the ability of the Enterprise's instruments to measure – apparently part of a destruct sequence, for seconds later, the buoy exploded. (TOS: "Spectre of the Gun")
James Doohan provided the voice for the buoy, but was uncredited on-screen. (Star Trek Encyclopedia, 3rd ed., p. 457)