- This page features the planet Edos; it is possible you may also be looking for the Edo species.
Background information Edit
Edos was the name of the homeworld of The Animated Series's Lt. Arex, as given in a biography published in a 1974 Lincoln Enterprises catalog. The biography noted that Edos was not a Federation member despite Arex being a Starfleet officer. It also noted that Edos was located in the Triangulum constellation, which was located at the edge of the Milky Way Galaxy, near the galactic barrier.
Edos was also mentioned as Arex's homeworld in 'Star Trek Log 4.
According to Star Trek Maps, Edos was the second planet in the 92 Trianguli-Rho system.
According to the RPG sourcebook The Federation, "Edo" was the third planet in the Epsilon Minora system. It was sometimes referred to as Sauria.
From StarTrek.com: "Home planet of Lt. Arex, the Enterprise navigator from 2269-70, and the tripodal Edosian race. Edos lies in the Triangulum constellation, on the rim of the galaxy. It is a jumping-off point for science teams studying the great energy barrier beyond the rim. It is also a peaceful planet which has never been involved in a war. The technology is as advanced as Earth's; the architectural style of its cities is strictly functional rather than aesthetically pleasing. The society has no class levels. All beings on Edos are equal.
Edos is not a member of the Federation. Rather, it remains in loose alliance with the Federation, coming under its agreements for trade, science, and aid in an emergency." 
Information from the Lincoln biography is not strictly canon, though the name of the planet seems to have been accepted in canon. See the full text of Arex's biography at Curt Danhauser's animated Star Trek site.