Rock Skulls

The Rock Skulls are amongst the toughest of all Ogre tribes. Each year at the Great Feast, it is inevitably a Rock Skull who wins the boulder-butting contests -- where Ogres try to split the most enormous of rocks using only their heads. Coincidentally, the Rock Skulls are also amongst the most dim-witted of Ogres and they have been tricked or swindled many times over by a great many opponents. Skarsnik, the Night Goblin Warlord of the Eight Peaks, notoriously hired a large contingent of Rock Skull mercenaries to fight in his battles against the Dwarfs. After being thoroughly bewildered during the contract talks, the Rock Skull leader ended up paying Skarsnik for the honour of aiding the Greenskins.

Source

 * Warhammer Armies: Ogre Kingdoms (8th Edition) -- pg. 21