Are horses social animals or do the travel alone?

No. Horses are herd animals and love equine company. Should you not be able to provide this, here are some things that can help:

-Sheep

-Dogs

-Cats

-Leave a radio playing (horses HATE silence)

However none of these are alternatives, just supplements. I really wouldn't consider getting a horse unless you can offer it company. Livery yards are places where you pay to keep your horse there. As well as having access to the yard's facilities, the horse will have plenty of friends!