In 1939 Fawcett Comics decided it was time to ripoff the OG superhero himself, Superman. They came up with Captain Thunder (in case you were ever curious about the lightening bolt). There was a last minute change to Captain Marvelous (too gay even for 1939), which lead to a last second change to Captain Marvel. Although in all honestly that was to reduce the size of the text bubbles.
It has to be said he was a roaring success. At one point Captain Marvel was selling 1.3 million copies per issue. No one in the industry even dreams of numbers that big today.
The reason it was a success wasn’t the hero’s powers, which were a complete ripoff of Superman’s but because of the backstory of Billy Batson, the news paper boy crippled by polio (back then everybody knew somebody with that problem), who with a single, magical word becomes an omni-competent demigod.
The stories were better and a young Jack Kirby was doing the artwork, so that was better too. What wasn’t better was Fawcett’s legal team. National (DC) Comics claimed copyright infringement on Superman. Fawcett didn’t exactly lose but past a certain point, the process itself became punishment enough for a struggling company. And Fawcett finally canceled the title.
And that is where things got interesting. Like I said, Fawcett didn’t have the best representation out there. Consequently, Fawcett let the Captain Marvel trademark (although not the actual copyright) fall into public domain. If memory reliably serves,* there were four of Captain Marvel’s running around at one point.