I’m sure you’ve all noticed that your horses prefer some horses to others . . . Zelda and Curly, for example, started mutually grooming each other very soon after they were introduced — instant BFFs. Zelda’s previous owner told me she’d never really bonded with other horses, and although Curly “gets along” with every horse she’s been put out with, I’ve not seen her actively engaged in friendship activities before.
My very first horse, Bogie, apparently had bonded so strongly with just one horse, that after that horse colicked and died, he wouldn’t go out with any other horses (safely) for years!
This video suggest that horses have a long memory for their friends, even when separated.