I say go for it. All the other 'royal' pony types are mash-ups of the other castes' features, so why stop here. Although I caution at making the race too large compared to ponies as that often leads to pony extinction in the histories as seen by the trouble with balancing buffaloes. Two such races would probably make that a guarantee, although restricting their ranges as well might help in that regard.
Oh, Nidokoenig, I also suggest allowing the relevant pony civ entities have the [BUILDS_OUTDOOR_FORTIFICATIONS] tag as well so they build castles. I don't really know if it actually helps them survive world_gen, but it definitely makes adventure mode more playable (noble quests and free, semi-competent followers). And its canon to boot.
As a side note, my Rainbow Pony Underlord adventurer passed 50 notable kills recently (not including the ones taken down by my squad) with another 40 misc. to my name. Haven't met a buffalo or zebra yet in combat, or at all on that note, but I have been fast traveling a lot so maybe I just need to slow down to meet one.