I do not dispute this, that theory is as valid as any other. Though I will point out that dragons fly to migrate so the inability to fly is a fairly serios problem especially if your guess about them not being accepted is right. About the only way I can see around it would be if another theory (that Spike is in fact Dragon royalty) is in fact correct (though in that case, you'd think the dragons in the Dragon migration would have shown a little more respect to someone who was visiby a member of the royal family). In that case, the wings may not be neccecary, as the king is simply carried by his subjects along the route That, or at some point Spike's innate magic grows to include levitation (Asian loong dragons manage to fly just fine with no wings after all, and that sea serpent serpent thing always looked a lot like one of those to me (loongs are after all water dragons, so the line between water dragon and serpent may be foggy.)
And it does look like pretty much EVERY animal in Equestria is highly intellegent. Angel certinly doesn't behave like a rabbit in our world would, nor does Tank, or Opal or Owlowishus. In fact Winona seems about the only one that does behave sort of like her species would here (I'm not sure where to assign gummy, since his primary character trait is basically not doing anything, he's less a pet than a living action figure for Pinkie). So I guess sentience is the wrong word, being able to talk seems the defining point.
I'm not going to try and work out which group are the most powerful, but it seems to me that all of the major groups seem to have some sort of agreement that a citizen of one empire is accepted as a citizen of the others. After all, there seemed to be no issue in having a Griffin as a canditate in an Equestrian run contest, and, had he won, he would presumably have been awarded his prize the same as any other (no, "you can't enter, you're not a pony) No one seems to think it odd to have a self help course run by a minotaur either, and Iron Will seems to be able to run his buisness without any species based legal trouble. It sort of goes back to what I said originally, it appear that, if you can think and you can talk, you have equal rights under the Equestrian law. Where friendship is king, there is no room for racisim (specieism).
"For I have heard the Song of War, and to me, it sounds just like "Please, Mister Custer.""
Cogito Ergot Sum, "I think there is something wrong with this rye bread." (Aplogies to Seafrol.)