It may surprise you guys, but horses are herd animals. They actually do tend to get along fine with each other without such management. Although I have seen frustration & anxiety *because of* keeping horses separated, causing over the top 'politics' when horses are finally introduced.
If horses(or people, dogs, etc) are kept in solitary confinement, esp if brought up that way, they can absolutely be socially inept and become antisocial & even aggressive with other Animals. It is a product of peoples intensive management practices & not allowing horses to be horses, not because 'it is dangerous to put horses together'.
And I'm not speaking from insular experience, but from general, worldwide practice. Its rare in the scheme of things, that horses are so micro managed as some people advise they must be.
So long as a new horse is introduced to others in an environment they know( let them explore paddock first, know fence line... And it's a large & open enough area for them to not get trapped, pushed through fences, etc, it is very rare for them to do each other serious injury, far from the terrifying & irresponsible situation it sounds like from some here. And they've already been together a couple of days already too kiger.
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And in a nutshell, this is precisely what animal behaviourist Marthe Kiley-Worthington says in her book "Horse Watch - What It Is To Be Equine." She shows in her case studies what those of us who grew up with horses kept naturally already know - that horses raised and run in groups exhibit far more cooperative than competitive behaviour, and that horses are far more likely to injure themselves and suffer health problems, not to mention social deprivation, if kept artificially apart as a norm.
I know of one stallion kept in isolation in his retirement, with a double fence across from others when in the paddock, and in a loose box adjoining a gelding at night (which the owner considers plenty social but the horse clearly does not), who after five years of retirement and years of pacing up and down the fence line growing ulcers, developed a habit of self-harming by spinning in a circle like a dog chasing its own tail, and biting his own flanks until they are bloody. What he does to himself is far worse than the odd scrape or bite you get when re-socialising horses carefully and appropriately. He is a really nice, friendly horse and would be fine with a gelding (or mare obviously, except he'd produce offspring). I've been encouraging the owner to have him gelded so he can be normally socialised - since he's not going to be bred from.
I did exactly that with my riding horse - gelded post-racing at age 11, when he was equally miserable from social deprivation and very aggressive to anyone and everyone over his fence. He was worked and gradually socialised and was running with a buddy after a year, and now runs peaceably as boss horse in a full herd including a pony and three donkeys (not bad for a "dangerous" horse many people told me they'd never even attempt to socialise).
Horses kept artificially may not learn how to socialise normally and may then need to be rehabilitated and introduced to each other with extra care and intermediate steps. Putting horses that grew up largely separated from others in together without such precautions can result in those injuries that many people then cite as a reason never to socialise horses normally at all - creating a Catch-22.
For re-socialising, large paddocks are preferable to small ones, preferably grazing paddocks as these give horses something to do and don't result in "food fights" - and check there are no blind alleys into which horses can be chased. It's best to introduce socially inexperienced horses gradually, by riding together, and keeping them adjacent for a while. A single or double electric tape is good as it prevents fence injuries (and ringlock or barbed wire or sharp-topped pickets without safety caps are definitely not right for such a divider). When hand-feeding newly socialising horses, separating them during feed times (and before the buckets etc appear) can prevent aggression until things become more settled down the track.
I've just mentioned some general basics here. The take-home point is that most problems with horses are actually created by humans.
I'm not sure if this photo will post as I am "away" but it shows two late-gelded ex-race stallions either side of a temporary double electric line. The dark horse in the centre is my riding horse, running with a little mare and a 30yo gelding, and three donkeys not in the photograph. The recently arrived chestnut pair in the background are the other late-cut horse and his first-ever paddock buddy, who is his full sister. They are all learning to hang with each other stress-free, and in a few months we'll do some direct introductions, with the view of them all running together. This kind of precaution is necessitated by the artificial way those three were brought up.
Here's the hyperlink should the above not work: https://www.flickr.com/photos/redmoo...n/photostream/
The change in demeanour and the obvious happiness of "battery hen" horses who have learnt to socialise is awesome to witness.