Ponies are made from three circles. At least when I do them. Think its from a guide. Head is biggest by a decent margin. Then pony butt, that's a fair sized circle, and chest is the smallest.

In theory, neck through chest to butt should end how with a simple line, like there is a sock stretched over the two circles. I'm terrible at getting legs the right proportion, think I tend to make them far too small and slight sometimes.