My headcannon is all ponies evolved with an inherent magical ability, unicorn earth and pegasi alike (its just that obviously unicorns have a much more focused amd stronger magical ability).  
Ontop of this, singing acts like a ‘magic overdrive’ alowing ponies to do things normaly impossable for a short time.
 
End result, ponyvillie, despite being made mostly of earth ponies, has enough magical potential while singing to change seasons in under 24 hours… damnit now I have winter wrap up in my head.

I think its musical visuals myself. though I’d like earth pony magic to be a bit more codified/strong.

Coincidentally, I read an MLP/Attack on Titan crossover fanfic, where Princess Celestia gave the Mane Six (and Spike) a few enchanted apple seeds, to take with them on their journey. They become full-grown, fully-fruited apple trees within a matter of seconds of being planted.

@Turkleson  

It happened in the Hearth’s Warming Eve episode, too. Smart Cookie just picked up a clump of dirt, and a little plant sprouted out of it.

Can’t it be both?

Magic needs no ’splainin.