The majority of foals, certainly in my experience here in the UK, are spending their first few months alone with the mother. Although I am aware of reputable horse breeders that turn out a number of foals and mums together to live amiably as a herd. Yet I have seen many individual horse owners segregate the mother and baby until weaning. Then between 4 and 6 months the foal is usually separated from the mother and put into a herd of horses.
Those were the words the livery used to inform me that her horse could not be part of a herd. This horse could not have a field mate. There would be no swishing tail to turn and face during the summer to deter the flies from bothering her eyes, or a restful sleep while the other horses stood guard looking out for danger. No mutual grooming would ever occur, and there would be no comfort from having a leader, or being the leader. This particular horse could not have a field mate because, in her words, her horse would kill other horses.