The character Boss Brolder did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Boss Brolder is a recurring boss in the Mario series, first appearing in Super Mario 3D World. As his name suggests, he is a large Brolder, and is presumably their leader. His rocks are noticeably more jagged than a regular Brolder, and there are also white crystals all over his body. His eyes and arms also appear to be made of lava.
Super Mario 3D World
Boss Brolder debuts in Super Mario 3D World as a boss. He is the boss of World 4, and is also encountered in World Castle and World Flower. He mainly attacks by trying to roll into Mario, damaging him. He can also spin around to send fiery rocks in different directions, as well as summon Brolders to help him out.
In order to damage him, Mario must damage one of the Brolders, putting them in a dormant state, then throw them at Boss Brolder. After being hit by three Brolders, he will be defeated, revealing the Warp Box that leads to the Flagpole.