What time point are you referring to as “the past”? Because the time you are asking gives VERY different answers.
If you’re asking about the period ~100 years ago back to about ~3000 years ago, the answer is that horses are useful to humans. Like humans, horses are pack animals; which means that humans were able to effectively move into horse pack leadership. Because horses are faster than humans for short distances, able to pull a lot more weight than humans, and relatively intelligent, horses provided a lot of extra utility for humans; and as a result, were spread by humans to just about everywhere.
This period only ended because of the internal combustion engine, which provides more power than horses, and is able to go faster than humans both over short and long distances; and the lack of intelligence is mitigated by the fact that it can entirely be controlled by humans.
If, instead, you’re asking about the period between about 12 000 years ago to probably 3-5 million years ago; the answer is that the general horse shape is highly adaptable. The general shape of a horse is relatively efficient, meaning it can survive on sparse grasses; but given large amounts of grass, it can make use of the extra energy to reach high speeds. It’s also an effective running fighter: most related animals (like elk) primarily fight with antlers – which exposes their head to predators. In contrast, horses’ primary attack is their hind legs, which means that they only need to pause momentarily while running away to kick approaching predators: “Fight or flight” is less of a problem when your answer is “but both?”.
This period ended possibly because of humans. Humans have been effective hunters for a lot longer than 12 000 years; but over the last glacial period humans spread out over a significant part of the world; and with the melting glaciers, expanded their population. Between about 12000 years ago and about 6000 years ago (when humans started looking for animals other than wolves to work with), humans hunted just about anything; which led to a wave of extinctions – primarily in the Americas, where animals hadn’t yet learned to fear the hairless bipeds without teeth or claws.
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