The horse species (Equus caballus) has been domesticated for thousands of years. While some wild horses exist, even they are descended from domesticated ancestors that escaped and thrived in the wild.
It's important to remember that "tamed" can mean different things. Some horses might be more difficult to tame than others, but all horses are capable of being domesticated with patience and understanding.
Perhaps you're thinking of a different animal? It's possible you're thinking of a species like a zebra or an onager, which are closely related to horses but haven't been domesticated to the same extent.