Let's get started! To begin with we are going to make some guidelines to help us get everything in proportion. So first draw a rectangle, as big as you want the back of the horse to be. This will help to get the legs and body in the right place. Make sure you leave room on the left of your paper for the horse's head and neck. Make the rectangle a little wider for a short-legged pony, taller for a leggy thoroughbred, or square in the middle.
Next draw two ovals like these. Notice how they fit closely together in the top half of the square. One oval is quite flat, for the barrel of the horse's chest. On the right a sloping oval for the horse's hindquarters.