>>/8352/
> Earth Pony
I would lean in that direction too. Though I am going to come up with lore related explanations anyway! 

> Discord.
Agree here though. 

 >>/8364/
Probably referring to this:
https://en.wikipedia.org/wiki/Duck_test

 >>/8355/
I like this logic. If I understand you correctly. The Earth Pony has unicorn parents (or at least a parent) and has to live with the same prejudices. 

I wonder if ponies born of wrong races for their tribes would try to rationalize themselves as being part of the bigger tribe. I assume naturally this would not have lasted if I go by what we see in the future. Though, far too much interbreeding existed for anything sort of a ruthless genocide to have wiped it from the population. Remanent populations I imagine would survive somewhere. 

 >>/8356/
> Also, Discord is, as Sunny points out, something of an unreliable narrator.
Also a good point.

> TBH I think the correct solution IS to destroy the crystals; they're containers not an actual source.
That would be a interesting take to turn the plot. Though, it leave it that Twilight Sparkle made a major mistake...