The horse has been evolving for 65 million years. Over the last few thousand years, since the horse has been domesticated, breeds have been introduced and developed. Many of these have been developed for specific tasks eg: pulling carts or more recently for racing. So it is likely that some of these breeds would cope better than others.
Our domesticated horses have maintained their herd instincts. They still have a well developed 'Fight or Flight' instinct, they have excellent hearing and peripheral vision. In addition, they have maintained their grazing habits. They still interact in the same way and learn each others 'signals' and so it is likely that a new horse/pony would be able to integrate. This obviously would be different for a horse or pony that had been kept on its own for a prolonged period as they may no longer understand other horse 'signals'.
So, I think that in theory some horses could adapt to living wild. If used to living with other horses they would certainly be well able to join and integrate with a herd. In time many would adapt to growing a sufficiently thick winter coat and if they survived this period of adaption would probably be fine.
Have you seen last week's video 'How to ... Wash your stable rug' ?
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