Bonebreaker is a mount added in Total War: Warhammer II.
A specially-engineered breed of Rat Ogre, the Bonebreaker has a braced platform strapped or bolted to its back for carrying a Skaven Warlord to battle.
This mount allows its character to do a majority of armour-piercing damage and imbues the Frenzy ability. This massively helps them deal out damage and gives them mobility to get around the battlefield.
Notably, both units who use this mount drop 20 armour (goes from 90 to 70) which is not super significant, but does mean they have less damage mitigation on top of being a bigger target.