The Wild Heart is one of the Overlords who ruled over all of Eberron during the Age of Demons. The Wild Heart was bound by the dragons and couatl after they jointly discovered the Draconic Prophecy, and the couatl sacrificed themselves to form the Silver Flame. If the Wild Heart ever had a name beyond its title, it was never known to the mortals that survived it being bound, perhaps it was only ever uttered by beasts.
The Wild Heart represents our fear of the natural world, and is the source of the curse of Lycanthropy. It's unknown where the Wild Heart is bound, but as the lycanthropes surged from the Eldeen Reaches during the Silver Crusade, it is likely that the overlord is bound somewhere within the Reaches. Just as Masvirik holds sway over cold blooded reptiles, the Wild Heart commands the obedience of all predatory animals (including predatory reptiles). Potentially the only reason the two didn't come into conflict with each other is that they're on opposite ends of the continent. The Wild Heart represents our fears of nature, not just our fear of beasts, and so plants and the weather also theoretically fall under its domain.
It is possible that the events leading up the Silver Crusade were a sign of the Wild Heart's increasing power, and that their release could have been imminent. We still don't know what put a stop to the lycanthropes, but it's possible that someone blocked the prophetic path that would lead to the overlord's release.