The Original King of Crime
This is probably the most obscure villain to be featured yet here. With only a handful of appearances, and only two being more than a cameo, this guy is for the weirdo diehards. First appearing in 1966, The Monarch of Menace was created by Robert Kanigher and Sheldon Moldoff. He had only one other full appearance in a Batman story in 1981, 15 years later. Mark Russell brought him back as a cameo in 2020, and he's made 2 other cameos since.
Introduced in Detective Comics #350, Batman describes the Monarch of Menace as the only criminal to elude him and escape into presumed retirement. However, the Monarch's son gets caught committing crimes using his father's alias. Batman and Robin use the son as bait to lure the Monarch back to Gotham and have him arrested.
When Batman goes missing after a battle with Ra's al Ghul, the Monarch resurfaces again. He capitalizes on Batman's disappearance and claims he has captured the Batman, using that lie to defraud other criminals. Eventually the Batman returns and ruins the Monarch's grift.
After this no one uses the character again for 39 years. Even then, these new appearances are barely worth mentioning. His recent cameos depict him as an Arkham resident, although he has not yet been named.
I think retconning him as just another insane patient is a mistake. Maxie Zeus has been treated the same way, and its a huge waste of potential. He is a thief, which is actually kind of rare in a Gotham full of mobsters, killers, and weird scientists. And while his royalty theme is similar to the Royal Flush Gang, it stands apart from the usual circus, mafia, and Alice in Wonderland themes in Gotham.
I've always thought the Monarch of Menace could have been a staple of the Batman villains gallery. He has all the makings of a strong pulp fiction villain. A seemingly successful thief, his true name never revealed, one of a very few to best Batman the Detective, the Monarch of Menace looked to be an allusion to the gentleman thief trope from pulp fictions. Like Arsen Lupin, Simon Templar or The Gray Seal, the Monarch of Menace could have been a recurring foil for Batman, the one villain who Batman can never get. To be fair, this had been Catwoman's role for a while, until she became an anti-hero and then Batman's girlfriend and pretty consistent partner. It's not too late for this character. In a continuity where all the other villains fall into a spectrum of mental illness from the very mild to the most severe, the Monarch could be a sane, intelligent, and masterful thief, a true polar opposite to the World's Greatest Detective.
I had some hope recently....
James Tynion introduced The Designer during his run in 2020. I thought for sure this would be revealed as the Monarch of Menace finally returning to Gotham. The Designer was described as being a legendary criminal mastermind, a shadowy architect of criminal endeavors, much like Sir Arthur Conan Doyle's Moriarty. I so wanted him to be the Monarch of Menace.
Imagine this retcon... The young Monarch of Menace thwarts Batman as is described in Detective Comics #350, but then never returns to Gotham. No children, no coming out of retirement, Batman just never sees a trace of him again. But... the Monarch builds a reputation underground as the Designer, and finally returns to Gotham to make his offer to the Penguin, Catwoman, the Riddler and the Joker as James Tynion writes in Batman #90. Joker thinks he kills the Designer and burns down his house, but the Monarch of Menace survives. This of course would change the rest of James Tynion's plot for Joker War, but still, the Monarch of Menace could have been awesome.
Sadly, it turned out to just be the Joker in disguise.