Earth Ponies - generally have a stronger, broader build than unicorns and pegasi. They are the most related to a realistic horse. Since they're connected to the earth or whatever, they have the builds to do the dirty ground work. I can imagine them having longer body fur too… don't ask me why.
Pegasi - Pegasi are made to fly and live in the clouds so they're covered in feathery fluff. The one word that came to my mind when drawing my oc in this style… FLUFFY! Pegasi are also associated with speed and athletism, so they tend to have a lean, athletic build with smaller frames. The best kinds of birbs are fluffy birbs.
Unicorns - Ah, the elagant but magical creature we call the unicorn. They are graceful but powerful horses, oftentimes veiwed as the most compelling out of the three races. In mythology, unicorns have a fragile, limber build with leonine tails. I decided to exclude the lionine tail for my personal style because I felt as if that would definatley be a trait that would have faded away with the ancient era of ponies. On another note, I had way to much fun drawing the curly fetlocks. I think now unicorns are my favorite breed of pony to draw now because of them, haha. As for the tails, I decided to hold on to a normal tail for now because most of the characters I draw aren't from ancient times. (I do plan to do something with it soon, though๐)
I didn't do alicorns yet, but I might decide to do that later.
Finally, this guide is strictly generalized. I still plan to apply the concept endomorphic, ectomorphic and mesmorphic body types. It just depends on the character and their genetics. For example, I have some earth pony ocs (ex. Coal Dusk) with a limber build and some pegasi ocs (ex. Adustio Flare) with a broader build, etc. Like I said, this is a generalized guide I plan to use down the road.
(this also gave me an excuse to redraw some of my old ocs, haha)