Ancient Beast

Ancient Beasts are creatures that roam the realm of Pandemonium. With Pandemonium being a harsh realm, Ancient Beasts have to be hardy to survive. Methods of such survival include feeding off magic that lingers in the realm and feeding off prey.

In-game
Ancient Beasts are a type of monster in Diablo III, found within Pandemonium in. Types include the following:

As they are not common Beasts, the items that have '+X% damage to Beasts' affixes will have no effect against the creatures of Pandemonium.
 * Barbed Lurker
 * Scavenging Tunneler
 * Scouring Charger
 * Primordial Scavenger