Answer

By Your Horse

Associate of the Intelligent Horsemanship Assoc. Sarah Weston says:

Putting horses together is a bit like matchmaking for single friends – you can go to great lengths to find a partner that is compatible and has similar interests, only to find that opposites attract. You are right to at least try, though.
On the whole, horses of the same sex, similar height and age, seem to get on well together. If your horse got on well with his last companion, it might be worth trying to find a similar one.
The main thing is to introduce them carefully – it pays to put them into adjacent fields for a few days to allow them to get to know each other. If they‘re both rideable you could also ride them out together so that they form a bond. And when they do go in together, it might be worth having their back shoes removed to reduce the risk of injury.
Fingers crossed that they get on well, but make sure you avoid them getting too attached to one another by regularly working them away from each other, and making sure that you are a good leader.