I have a very social horse whom I have thought about getting her a companion pet.
Some of my concerns would be;
1. Must get along with horses and not worry about them fighting for food.
2. Must stay in standard post and rail fencing
3. Must be able to tolerate cold winters
4. Must be able to be left if gone to a show or must be able to travel well to go to the show
5. It would be nice if they were tall eneough to reach an automatic waterer so I didn't have to water seperatly but small enough to not be in the way.
6. Fairly low maintenance in care and costs.