Known notes on the Horn of Valere:
Okay…we don’t really know. No one really knows. It’s an ancient artifact, probably not a ter’angreal.
Robert Jordan confirmed that Rand was a hero of the Horn. If a hero of the Horn was already born when it was sounded, they would not be affected by its sounding.
The Horn of Valere was known in the Age of Legends, though it was an artifact of an earlier age, but it was never used in the Age of Legends. In part, this was because there wasn’t any need in an Age that knew universal peace, but also it was because what it could do was considered a sort of myth by most people in that Age. No one who is serious spends time trying to test out whether a myth might be real. (Seen anybody sacrificing a white bull to Jupiter lately?) And once the Dark One touched the world, before the War of the Shadow actually began, the Horn was among the items lost, and thought destroyed, in the first rush of mob violence, terrorism etc. So it wasn’t available for use then even had someone wanted to try. It was later recovered and sealed up with the Dragon Banner because along with the Foretellings that made up the Prophecies of the Dragon was one saying that it must be.
The Horn of Valere, as Maria said, it was created by mortals—we know that; Jim has said as much publicly—and the Horn was created in the Age before the Age of Legends, or at least one Age before; it was not known how far back.
Oh yeah. So now we know that the Wheel even weaves inanimate objects into the Pattern, and makes use of them as it wishes.