I'd taken some photos of a horse display in China (attack of the Mongol horde) but had real problems when I came to draw the horses. So I got a copy of "The Artist's Guide to Animal Anatomy" by Gottfried Bammes and had a go at some anatomy/structure drawings.
First sketches of the stages of a gallop.
Back and front legs
Then some practice from a book on Arabian horses
When it came to adding a rider I couldn't get the proportions right and ended up with something more like a lego figure.