If you're not up on old comics, I'm going to blow your mind right now: Peggy Carter didn't actually appear in World War II-era Captain America comics at all. Peggy was added in flashbacks in the 1960s, when Captain America emerged in "modern day," because Stan Lee believed Captain America's backstory would be so much more tragic if he had a lost love he couldn't have been with because of the turmoils of war, and who was separated from forever.
In the forties, though, Captain America had a girl that did more or less the same thing the later Peggy would, Betsy Ross, and by 1946, Cap invited her to be his sidekick, alongside Bucky.
Yes, you read that right: the predecessor to Agent Carter was a lost, forgotten secret female sidekick of Captain America.
Yes. She showed up in the background of an Ant-Man story set around a Retirement Home for Old Heroes as late as the 2015, which meant she went close to 6 decades without a return appearance or barely an acknowledgement of her existence. Because this character steps on the toes of someone way more important (by this point) in the Captain America mythos, Peggy Carter, as well as the state of Captain America's post war stories in continuity (don't ask), it's unlikely we will ever see anything like a return of this character.
The Metal Master, first seen in Incredible Hulk #6 (1962) was an alien criminal who combined Magneto's metal control powers, arrogant poses, and disdain for ordinary homo sapiens, all together with Professor X's baldness. The Metal Master was a criminal thrown into space as a punishment, who wandered for ages looking for a planet to rule...unfortunately, he picked the planet with the Hulk on it.
It's pretty uncanny, no pun intended, how close his M.O. was to the later Magneto's: basically, float over military bases with his arms crossed over his chest, posing like a tool, all while arrogantly proclaiming that all of earth's weapons are ineffective against him, and turning the army's weapons back on them when they try to attack.
The interesting differences is that 1) It's heavily implied that everyone on the Metal Master's planet have his powers, 2) his power is straight up metal control, which means instead of levitating things telekinetically, he moves metal objects almost like a liquid.
Basically, yes. Nobody in Marvel is ever 100% forgotten, though: in 50 years, he showed up in one issue of Rom the Spaceknight, obsessed with the idea that his metal powers don't work on Rom's living metal robot body (scintillating motivation, there, guys) and he came back in the Kurt Busiek crossover, Maximum Security, where Earth became a prison planet to contain all the evil aliens, which included alien guys we haven't seen in a while, like the Asparagus People Jean Grey wiped out as Dark Phoenix.
The first Marvel Comic Stan Lee wrote that had mutants in it, Amazing Adult Fantasy #14, didn't have the X-Men in it at all, but instead featured a dope with the dope name of "Tad Carter."
What is a mutant, by the Marvel Comics definition (not the proper scientific definition)? They have strange and unusual powers they are born with thanks to atomic bomb testing and that make them a next step in evolution, and they are feared and hated by normal humans, who try to kill mutants with panicky mob violence.
All of the above traits were found in Amazing Adult Fantasy #14, featuring Tad Carter. It's actually incredible how much this is like X-Men. He demonstrates his mutant powers, and even though he wants to use them to help mankind, everyone is too fearful about him, and in the end, he is contacted by a greater community of mutants who invite him to join them.
There's even secret mutant communities far away from humans, a leader who contacts you with mental projection, and a lecture about how "humans fear what they don't understand" using exactly. Those. Words!
The amazing thing about Tad is that, thanks to Spider-Man co-creator Ditko's art, he looks just like Peter Parker. Seriously, just based on the image, guess who this is.
(It's actually Tad Carter.)
Incredibly, Peter Parker's look was based on Tad Carter, not vice-versa, since Tad came first. Amazing Adult Fantasy, after the issue that Tad appeared in, would be renamed Amazing Fantasy, and would be the first appearance of Spider-Man.
Yes, for decades, until John Byrne tried to incorporate Tad Carter and the community of mutants he was called on with what we currently know about the X-Men. He brought Ted Carter back in a big way in his X-Men: the Hidden Years. The secret community of mutants he lived in was basically a creepy cult, which should be obvious from their matching jackets.