Tkon Empire
The T'Kon Empire was an interstellar civilization that predated the development of space flight on Earth. The civilization had extremely advanced technology: an automated outpost was able to immobilize both the USS Enterprise and a Ferengi starship easily with planet-based tractor beams[1]. According to Federation records, the T'Kon Empire supposedly had the technological means to "move stars", but since the Empire fell and the race became extinct after the supernova of their home sun, the claim is somewhat dubious.
No living T'Kon was ever seen, but if the figure used by their automated outpost, "Portal", was representative of the race, they were humanoids. Portal had some degree of telepathic ability, which may or may not have been typical of the T'Kon themselves.
The T'Kon Portal only appeared in a single episode of Star Trek: The Next Generation, making them a one-episode alien.