Blastdown, I'll just say that the first paragraph of your guessing is correct (although you haven't explained any reasoning behind it) and the second paragraph is way off (from what I've heard).
Well, it would be easier to explain if there wasn't spoilers but Im just gonna say, he got corrupted or changed somehow.
Maybe he was different before... Plus they never actually show or say what race he is, even the wiki says so.
Incorrect. In Part 1, Discord is referred to as a "Draconequus" (variable spelling; sometimes Draconequis), a creature with "The head of a pony and a body made up of all sorts of things" or something to that effect, as explained by Cheerilee.
Therefore, his race is solidified, and he better not show up as something else. That would be a very stupid retcon.
(That, and the entire idea behind one of my OCs depends on the Draconequus being an actual species)