Aliens are just like us if you think about it, man.
Atlanteans:
First things first: these are not actually Atlanteans, at least not yet. As far as I can tell, the word “Atlantis” is not employed in any Namor appearance from 1939-1941, and his people are collectively referred to as “Sub-Mariners” just like their prince. We are left with one of our signature Vexing Conundrums: just when do the Sub-Mariners become Atlanteans? Will we find out before we learn the location of Camp Lehigh or before someone says “Keystone City” in print? Only time will tell.
Given that they are only later assigned the status of being descended from the humans of old Atlantis (and then unassigned from it if I understand correctly – my interest in Namor has never been great enough that I internalized a lot of his lore), it makes sense that they are much less humanoid than a lot of their peers. They range from being full-on fish-men like Emperor Thakorr to being essentially human-looking but for their huge eyes. And not just human looking, as Namor’s own parentage attests: they can also successfully interbreed with humanity! And it’s a close enough hybridization that he’s not completely infertile, though the fact that he has only one conventionally-conceived child is a bit telling.
One of the hallmarks of Marvel’s Atlanteans going forward is of course the fact that they all have blue or green skin, which is fun because it appears to have originally been a stylistic choice on the part of Bill Everett to indicate that they were underwater. Frankly I think that it was a good development: I have no desire to see Emperor Thakoor rendered in shades of pale pink. (Marvel Comics 001 1939)
Balticans:
Represented here by the villainous Rathia, the Balticans are an Atlantean subgroup that live in the Baltic Sea (natch). They might be a different colour than regular Atlanteans, but given the quality of the available scan and the unreliable nature of modern recolouring it is very hard to say for sure. (Human Torch v1 005b, 1941)
Cygnusians:
The Cygnusians and their enemies the Draconians are initially set up to star in a little allegorical story about institutionalized racism and the arbitrary nature of warfare, but things get derailed pretty quickly due to the fact that both Queen Luxor of Cygnus and Queen Feros of Draco are women and cheap misogyny laffs are easy. Rock Braddon of the Space Legion ends the war by arranging a queen-on-queen catfight. (Crack Comics 011, 1941)
Martians:
After dealing with all the crazy bullshit that lurks beneath the Earth’s surface in 50032 CE and doing a bit of civilization-rebuilding, Blaze Barton and his romantic/ adventure partner Teddy set out to establish relations with the people of Mars. As they arrive they (and possibly also the Martians) learn that there is a second, bearded population of Martians who live in vast caverns under the planet’s surface and eschew modern technology other than rocket planes and planet-wide smoke screens. These Dark Martians capture Blaze and Teddy in order to hold them for a ransom of lamp oil from an Earth that is probably not equipped to deliver it, given the recent global disasters it has suffered, not to mention the 47 thousand-odd years since the collapse of the whaling industry.
The main problem with being a low-tech antagonist in a science fiction world is that you are only effective as long as you can keep your opponent off-balance, as the Lord of Martianland learns when he gives Blaze a moment to breathe and finds his men completely outclassed by Blaze’s ray gun and working knowledge of electricity. The king himself is finished off with a low-tech right hook, and his people opt to learn about new and exciting sources of light. (Hit Comics 005, 1940)
