If you are looking for The Edo, a humanoid species native to Rubicun III, please go here.

The Edosians are a race native to planet Edos in the Alpha Quadrant. They bear some physical resemblence to Triexians, though Edosians are considered a more animated race. (NF short story: "The Road to Edos")

Arex was identified as an "Edosian" in Alan Dean Foster's Star Trek Logs novelizations, based on never-aired background information from TAS. That he was actually a Triexian was established in the New Frontier series, and the root of the confusion touched on in "The Road to Edos."
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