The Scavenger is the creature inside the cage mentioned in the Dune Thresher tale. The hunter unleashed three of these Scavengers into the sand as bait for the Dune Thresher. They dug out of the Dune Thresher's stomach, before dying to the intestinal acid, effectively killing the Dune Thresher.
Sounds like you will need a few stamina potions to outrun these pestering menaces. eh?
The Diablo 3 website has also been updated with seven new screenshots