Stamina
Stamina is the ability of how long your horse can gallop.
Lower Stamina Horses.
A horse with low stamina levels (Around 40 or Below) will tire more quickly than that of a horse with higher stamina. With lower stamina, your horse's energy bar (seen above the inventory hotbar when riding your horse) will deplete and turn dark red all the way quickly. This is due to you pushing your horse's low stamina to the limits. When a horse's stamina is completely depleted, it will slow down to a crawl. Or perhaps, buck you off.
Higher Stamina Horses
A Horse with high stamina (Around 60 or above) will be able to gallop for a much longer period of time than a horse with low stamina. A high stamina horse's energy bar will deplete much more slowly than that of a horse with low stamina.
But true with all horses in Equiscraft: The longer you have them galloping, the faster their stamina and energy will deplete.
Tip: To maximize a low stamina horse's stamina, avoid going faster than a full canter, and do not make the horse gallop.