Energy

The horse's energy stat is one of the four care stats that you will maintain on the horse.

Description
The energy stat on all horses affects whether the horse can actually perform at it's full capacity. When a horse's energy falls under 10%, it will slow down to a trot and struggle to fully jump. Maintaining the horse's energy is vital as the horse cannot speed up or jump whilst it's energy level is under 10%.

Maintaining Energy
A horse's energy level regenerates when it is not in motion whether you are outside on the field or in the stables. However, energy generates faster if your horse is inside the stables rather than outside as stated in the "Power Nap" quest.

If you wish to bring your horse back with it's energy under 10%, it may be faster to simply walk the horse using a lead from your inventory rather than riding it.