Iím a 70-year-old life-long artist and horsewoman; horseís, of course, are my favorite subject and Iíd just advise you to study horse anatomy and do lots of sketches. Iíd agree, the hindquarters of the horse you drew is too small compared to the front; also the barrel (the main body) is too thin...just bring the back up, coming off the withers a bit higher. That shows especially in the first drawing. The right front leg needed to be bent......I donít know if this helps but it might be a good start. Iíve made some corrections in red on your picture..... Happy drawing!