Warning: LONG POST AHEAD
Earth Ponies:
They are able to master elemental magic of a specific type.
Pegasi:
They are born with air magic in them.
Unicorns:
They can call up magic at their whim.

I swear this is awesome
Plotline:
There are four seasonal ponies. Rainbloom, Sunswirl, Leafglide and Snowdrop. The dark magic that infected Luna is inside Snowdrop, transforming her into Snowstorm. Banished to the highest cloud in the sky, Snowstorm didn’t strike until a thousand years later. And in the midst of spring, an eternal winter strikes.