Except that Articles of the Federation refers to Edoans and Triexians as "two species".
I reckon that the Triexians are a lost, then refound, Edosian colony.
I never understood why they changed "Edoan" to "Edosian," and I understand even less why Peter David coined "Triexian" instead of using the established species name. Let alone why he came up with such an implausibly on-the-nose name -- ooh, a tripedal species named Triexians. Isn't it bad enough having cat people named Caitians? As far as I'm concerned, they're all Edoans.
The Caitians (and the Lynx constellation) were spelled out, as relating to M'Ress, in the TAS Writers' Guide (same as the TOS guide but with a few extra pages - and even a little hand-drawn map showing the location of Cait in the galaxy), and Bjo Trimble used that document, plus scripts and episode viewings, in her "Star Trek Concordance", even though no script with that M'Ress background info was actually completed. There were supposedly plans for the info to be in a script that did not progress to final draft.
Arex was given neither a species name nor planet name in the TAS Writers' Guide. It seems the terms were coined by Alan Dean Foster for the "Star Trek Logs" -- or whomever wrote the Arex and M'Ress biography pamphlets sold by Lincoln Enterprises. "Star Trek Maps" (Bantam 1980) gave Edos the stellar nomenclature of [edit…] 92-Trianguli-Rho. (I suppose PAD continued that gag with Triexian?)
I am guessing that Peter David used the material from the Concordance, rather than the "Logs" or biographies. Hence, he had "Cait" and "Caitian"" but not "Edos" and "Edoan".
I learned about the existence of the TAS Writers Guide from former TrekBBS member,
The God Thing, who used to post here. He found it quite randomly in a reference library in the USA.
I could be misremembering, but I thought we first heard "Edosian" in terms of the orchids... But then everyone started using "Edosian" for the species name too.
Yep, that is my memory of it, too.