Interesting discussion time!
So my friend got that "Twilight Sparkle: Teacher for a Day" iPhone app, and it actually goes into some of the history of Equestria. (seriously, it's weird) It's an official product so it counts as canon.
For instance, did you know at one point Earth, Pegasus and Unicorn ponies where enemies? They didn't associate with other races and sometimes found themselves at odds with them. But then there was a great blizzard that drove them to work together to build Canterlot to survive, and all the different races of ponies lived in harmony.
I'm surprised more people haven't noticed this yet, there's some great story potential here.