There are a few horses that do well living alone, but -since horses are herd animals- the majority of horses will do better in the company of a friend/"herd".
However, that "herd" doesn't necessarily have to be a herd of horses.
It could be a goat, a pair of goats, a donkey, some sheep, a llama or two, a cow, a camel...really any 4 legged prey animal with a herding instinct can potentially fill the herd need.
My late mare actually did better with a pair of goats than she ever did in a herd of horses. One goat was dominant over her and the other was submissive - I think my mare really liked filling that middle spot.
Anyway, goats can be hard to contain but I really haven't had many issues with that. They do like to slip through the fence and visit the pasture neighboring their pasture, but they rarely escape-escape. I think the fact that they have 6+ acres to roam in their pasture really helps.
My mare only passed a way a few weeks ago and I've been SHOCKED at how little goats eat and at how absolutely cheap they are to own. I knew they didn't make much of a dent in my horse-budget but I had no idea of how little a dent they actually make. In the last two weeks, plus daytime pasture, they haven't even finished off two 60lb bales of hay. They legitimately eat one 3lb flake of grass hay/night [they're out on grass, foraging, all day] and they're totally happy!
And they aren't small goats. One is about 90lbs, the other is closer to 200.
But yeah, most horses need a companion or two to do well but that companion doesn't have to be nearly as large or expensive as a horse is.
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.