Horses produce manure which after 6 months or so can be used as fertilizer. Sadly, they produce lots of going to be fertilizer which mostly can't be sold for diddly squat anyways. But I love horses.
Manure used too fresh will burn the area trying to be fertilized. Had horses my entire childhood and shoveled lots of byproduct. Learned to ride on a Shetland pony. If you could ride him, you could ride anything we owned.