Elder Gods, the creators of the Horn.
The otherwise unnamed Horn is an Elder God artefact about which nothing significant is known. It was found by Guthix on Gielinor shortly after he discovered it, in the First Age. It was one of the many artefacts that granted him the power he was known to have. Along with the other artefacts in Guthix's possession, it was hidden by him before he went to sleep at the beginning of the Second Age. Its specific powers or whereabouts are currently a mystery, although it is one of the horns that are mentioned in-game. there has been speculation that it could be the Frostenhorn or the horn in Quin's possession. The former is confirmed to be false, the latter has not been commented on.