We got your aliens, plus some so forth.
Moon Men:
The Moon Men of 2050 CE seem to be just another part of the future Solar System society until they enter an alliance with magic super-crook Sarku so that their Moon King Tan Tan can meet his historical crush Elena. This of course brings them into conflict with Mr Mystic, as Elena is his fiance and he's just not that into her being kidnapped into the future.
The Moon Men also have very upsetting feet. (The Spirit Section, 1 September, 1940)
the Alligator-Men:
A lot of the Earth-based entries on the big list of aliens and such that I refer back to to while make these could best be described as "human offshoots," little pockets of humanity that have developed in isolation until they are something other than regular. I had a little debate about whether to include the Alligator-Men because they didn't seem other enough, but then I came to my senses. A whole society of guys who live in a cave somewhere in Brazil and dress in what must be wildly smelly lizard man costumes 24/7? Even if in-story they turned out to be a bunch of cosplaying accountants (spoiler: they didn't) that's worthy of comment. (Amazing-Man Comics 017, 1940)
Ape Men:
And speaking of isolated populations of weirdos, ape men/ape-men/apemen and other missing link-style hominids are some of the most common. This particular group are from Malaysia and seem to be reasonably chill until Dr Fung and his pal Dan show up while following rumours pointing to the location of a lost heiress. Things get a bit too macho (and the old question of just what "white" means in 40s comics rears its ugly head once more) and Dr Fung and Dan have to shoot their way out. It's all very rude, frankly. (Wonderworld Comics 005, 1939)
Ape-Men & Lizard-Men:
The Diamond Empire is a hidden civilization probably somewhere in Asia, located in an improbably large inaccessible region hemmed in by mountains - there's a reason that so many hidden kingdoms are underground, after all. It's a bit more plausible to have an implausibly large area underground and hidden than as a conspicuous blank spot on a map.
In any case, the lands around the Diamond Empire are like most other terrae incognitae in genre fiction, in that they are populated with a variety of humanoid civilizations, including Hawkmen, Batmen, and today's focus, warring nations of Lizard-Men and Ape-Men. The Lizard-Men sadly get the short end of the stick characterization-wise, being sneaking, cowardly ambushers with a uncomfortably horny king, but at least they have very cool faces - just love a black eye with a white pupil.
The Lizard-Men get beaten up and pushed in a river by their ancient foes the much less cool Ape-Men who then themselves capture the the Rocket and the Queen of Diamonds. These Ape-Men are just what they are billed as: apelike humans, though the yellow heads and weird dents on top of their skulls set them apart slightly, even if they look dumb.
There's a lot of malarkey involving the Rocket taking on an Ape-Man named Ungar for the role of tribal chief, and we get a glimpse of the Ape-Men's idea of how a trial by ordeal should work as the Rocket is made to enter the Cave of Horrors to prove his mettle, but eventually the majority of the Ape-Men just end up dumped into a chasm to their dooms. I assume that the Lizard-Men, merely damp, were very smug about this. (Pep Comics 004-005, 1940)
