Horses started to appear in art more than 17,000 years ago, in the prehistoric Lascaux cave paintings found in France. Of the 6,000 images found there, 364 are of horses.
When I was a kid I spent hours drawing horses, cutting out pictures of horses, making horses from paper tubes. I found some of my early drawings a few years ago. Picasso I am not! My pictures more resemble the cave paintings.
Here are some videos that will get you started along a more proficient route.