Explanation: The Roman Emperor Caligula, sometimes said to be mad, once threatened (or joked, depending on who you ask) to make his horse consul, one of the two leaders of the Senate. Caligula was a tyrant, a dick, impulsive, and petty, but one can argue he wasn't mad. Just... well, a tyrant, a dick, impulsive, and petty. And with a cruel sense of humor.
The Roman Emperor Elagabalus had a much more discernable form of madness - youth mixed with ultimate power. Elagabalus was a teenager when they (as their gender identity is a matter of perpetual debate) were oh-so-wisely granted absolute autocratic power over a continents-spanning Empire by the political machinations of their grandmother. Two of their more notable offenses were proclaiming themselves to be the Syrian sun god, and having young women hitched to their chariot like horses to pull them around Rome, whipping them all the while.
While you can say teenagers fulfilling all their most wild teenage fantasies with no one to stop them isn't 'real' madness, it seems closer to it than flexing on Senators and daring them to defy you as a petty show of power!