(Detective v1 031-032, 1939)
The only reason this guy’s minor is that Batman shoots him dead in 1939. I know he’s made a few reappearances over the years but mostly they’re retellings of that first story. Also I guess they’re calling him the Mad Monk now? Weird.
My point is that Batman deserves a vampire nemesis so they should BRING HIM BACK.
ADDENDUM: it’s very strange to me that in Batman: Red Rain it’s Dracula who vampires Batman and not the Monk, an established vampiric Bat-villain.
(Adventure 044, 1939)
Another early Sandman villain. A fairly standard master of disguise who was later reworked into an axe-wielding maniac in Sandman Mystery Theatre.
(Adventure 040, 1939)
Standard masked crook, remarkable only for being the Sandman’s first foe. Shows up later in one or two crowd scenes.
This story is also notable for being the only time Golden Age Wesley Dodds tucks in a little doll version of himself before heading out as Sandman, a charming bit that was later adopted as an ongoing thing in Sandman Mystery Theatre.
(Action Comics v1 001, 1938)
The Tigress was both Zatara’s first foe and his only real recurring villain, showing up a dozen times over 3 years. She acted in the role of what was occasionally called a “friendly enemy”: a villain of the opposite gender who the hero has a soft spot for, implicitly or explicitly because they’re horny for them. A Catwoman situation, in other words.
Like Catwoman and Batman, the Tigress occasionally teamed up with Zatara against a worse foe, and Zatara frequently let her escape at the end of a case out of horny sentimentality. Unlike Catwoman, the Tigress was absolutely a multiple murderer, running armed robbery, assassination and extortion rings at various times as well as acting as lieutenant for several other super-villains such as the Mad Lama.
Ultimately, Zatara caught the Tigress for the last time and seemingly persuaded her to reform. Whether it stuck or not, she never appeared in a comic book again.
(Detective v1 019, 1938)
Nemesis of Inspector Kent of Scotland Yard with maybe three appearances – but he’s the kind of dreaded evil mastermind who shows up already the most wanted man of all time, so he feels more significant.
(More Fun 014, 1936)
Another Dr Occult foe (and thus another early Siegel and Shuster creation), Koth is a fairly generic magic guy, for all that his origin involved being a magic alien who swore to destroy humanity when his companions were slain by cavemen.
Unlike the preceding minor villains, Koth has gone on to appear in more modern comics, though whether any of them were set in the modern day as opposed to being period pieces is something I’m not sure of..