Best practice

Horses are herd animals and should not be housed alone. 
They do best when there is at least one other horse with them.

Needs intervention

If it is not practical to have a companion horse, goats or cows can also be suitable companions.

Humans are not sufficient companions

Immediate jeopardy

Intact stallions should not be housed with mares.  Stallions as young as one year can successfully breed.  Stallions should be castrated or housed separately with other males.