BoneGnash, along with GhashBone, is one of Motor-Runner's guard dogs in Vault 3. Both of the guard dogs will turn hostile as soon as you harm Motor-Runner, regardless of whether or not you are hidden. This may cause the other Fiends in Vault 3 to turn hostile as well. It is suggested that you close the door and kill him, Motor-Runner, and GhashBone in rapid succession.