If I'm making a species that's still decently horselike, how should it look like?

You should take parts of what you want. Horse breeds are different from each other because of how they slope in different places, bulk, long/short legs, dished face/Roman nosed, hoof width, eyes, tail set, etc.

For any equine, remember their species, and characteristics of said species. Both a horse and a pony are equines, but a horse is not a pony. Ponies generally have a shorter, stockier build. Their stomachs and chests drop further, and they usually grow a thicker coat. For the specific pony you want to draw, they seem to have rather rectangular heads, and a straight profile. There does not seem to be much variance in head shape. No dishes or roman noses. They seem to be slightly post legged, but that is a minor detail. They have long tails, and short manes. Also, I forgot to mention this, ponies are very wide. Not fat, but literally big boned. Their rib cages span out further than horses. Other than these features, they should be drawn as a normal pony/horse. They do not have any major characteristics, such as a draft horse or an arabian.

You should look up the pony, study it and then draw it! It doesn't matter if your first drawing is not very good, keep on drawing it, and drawing it, and drawing it, and it will be beautiful!