The horse's legs: first add lines to fill out the legs, with a big inverted triangle for the upper part of the back leg, all the rest pretty straight.
Head and neck: join the square at the nose to the cheek-circle to form the horse's head. Add the ear(s). Draw a curvy line joining the cheek to the neck, making the bottom of the neck-triangle a little fatter too. Draw a curved, arching crest over the neck.
The body: square off the horse's chest at the top of the foreleg. Join the chest and hindquarters on top and underneath with lines that curve in slightly.