I have used a billy goat to keep my stallion company over the winter months one year. they were in stables next to each other. The stallion actually really liked the billy, they were best buddies
Could even turn them out in a field together.
I am currently working with two foals one weaned about a month ago and the other about a week. I am lucky and have a spare gelding as a field companon. The older of the foals remained inside with the goats for company for two weeks while he was haltered and had some basic training (I have a gated system). The goats encouraged the foal to eat hay and feed times were the same so he copied what they did. He is now in at night with goats and out in day with gelding. I feel this resolves some of the separation issues that you can get with ponies not wanting to leave there field friend.
The younger foal came in a week ago and is in the barn 24/7 with goats, but has the other foal to talk to at night when he comes in. When it is time for the younger foal to come out I put the older one in a field on his own some of the time (he can still see the others) and leave the younger one in the day with the gelding and night inside with goats. Then just move them about so they are used to all combinations.
Having another pony to talk and train the foal is really important, the gelding sorts out any issues that come up so that the foal is that much more submissive when I have to deal with him/her
Hope this helps you a little
or maybe you think I am talking rubbish
but it is working for me
Diane