Moderate sunlight is necessary for the health of almost every mammal, including horses

The Sun is Healthy for Horses

Horses are like people.  They need sufficient sunlight for proper vitamin D production and bone formation.

"Horses need some free time in daylight every day for optimum health and a fully balanced energy field.  If your horse is confined becasue of veterinary treatment/instruction, see if you can use a stable where the horse can put its head over the door into good bright light and use a light airy stable. Check with your vet in case of contraindications” (Coates, 2001).

Remember that Horses Need Shade Too

Remember to provide shade for horses that are turned out so they can rest out of the sun, if needed. 

References

Coates, M. (2001) Healing for Horses, London, UK: Edbury Press.