I'm not sure why I did this, but I wanted to study how MLP handled species ratios throughout the years, and built a spreadsheet:

Couple interesting patterns here in how the franchise handled it:
*Gen I: Two of every species
*Gen II: Just Earth ponies, apparently they abandoned mythical horse spinoffs
*Gen III: This time there was a pair of characters who had the gimmick of "is a pegasus" and "is a unicorn", with the rest of the cast being Earth ponies.
*Gen IV: Went back to two of each species (far from the only thing Lauren Faust stole from Gen 1, you can tell she mostly grew up with Gen I and ignored where the franchise had gone after she grew out of it)
Which leads to Gen 5, which is weird in that there are only 5 protagonists, it's missing the second unicorn. Maybe they'll add one later or she got cut for other reasons? It's also the first generation to have a stallion in the main cast, which is kinda crazy