Herkers are large OverCharge Drinkers, who have the ability to throw other mutants at the player. They're much larger than a standard OD, thus causing much more damage. Additionally, one of their arms is abnormally large, with an excavator's bucket at the end of the hand, making them a great threat in close-quarters. It is also unknown why the Player dubs them Herkers as made in a comment by the Player themselves. Their excavator arm could lead to the fact that they were in a construction yard or building site when they mutated.
Gallery
A Herker along with some OverCharge Drinkers
A Herker running towards a player
A player flying away after getting hit by the Herker's Excavator Hand
A player shooting at a Herker