ALPHA CENTAURI

Non-humanoid green skinned sentient alien lifeform. It possesses a large one-eyed head, and a body equipped with six tentacles (plus most likely a not witnessed fourth pair used for locomotion). Each tentacle is equiped with prehensile pincers. It has a high level of intelligence and itr vocal system allows it to speak the human language. It has a fairly long lifespan, given the fact that the one serving on Peladon was in office for more that fifty years. Its civilisation is most likely highly advanced and peaceful.

The member of the species that served as one of the Federation's delegates to Peladon helped the Third Doctor unmask traitors to the Peladon crown and thereby facilitate Peladon's accession to the Federation. Fifty years later, when the Doctor returned to Peladon, he had been promoted to the title of Federation Ambassador.

It is never made clear if "Alpha Centauri", being the name of the alien, also stands for its planet of origin.

Source: Doctor Who, "The monster of Peladon", "The Curse of Peladon"