Exactly. If there really was a centaur discovered somewhere, the whole world would know it.
That brings up an important question: IF (emphasis on if) some kind of half-human was found somewhere, and if it is as intelligent as us, what would humanity do? Would they allow the creatures to live amongst us, put them in zoos, or extinct them over time, like what happened to the indian tribes in America?
The Indians didn't necessarily go extinct. They were beaten from the outsiders, for their technology was advancing to guns, boats, and swords. While the Indians were still using wooden/flint tools, and bows. Some turned into slaves, others were executed, or banished from their land. But after a lot of time (and I believe some conflict or uproar from neighbouring areas claimed by other countries I believe) some ended up joining this new society. There still are people today who's ancestors are Indians from North America.