The Slag Brothers, Rock and Gravel Slag are twin Cavemen brothers, driving a wheeled boulder, the Boulder Mobile. The Slag Brothers sometimes reconstructed their car from scratch just by using their clubs on any large boulder that was available. Like the Gruesome Twosome, the Slag Brothers can summon up appropriate creatures – such as Pteranodons – to help them. The Slags also speak by combining stereotypical caveman language with normal English, e.g. "Raga-radda, wheel gone. Get new one!" In one episode, the Bouldermobile had a "flat" tire, when a wheel turned cubical. In another episode, a wheel fell off, and Gravel Slag accidentally made a square wheel from a rock, to which Rock Slag responds, "Rah Dummyhead! You make square wheel!" They can accelerate by hitting the car (or at times, each other) with their clubs. The Slag Brothers' character design was re-used for Captain Caveman.
The Slag Brothers were originally voiced by Daws Butler and Don Messick, respectively and currently by Jim Cummings.
If one were to count up every last winner from each episode, you would find that the overall winner for most 1st places is a tie. But according to real world race ruling statistics, should the individual wins be accumlulated, the Slags themselves would hold 1st place, Rufus Ruffcut and Sawtooth would be 2nd and the Ant Hill Mob would take 3rd place.