It's not terribly surprising that there are some characters in both of the Big Two that seem... similar to one another. Sometimes it's not really intentional, like Swamp Thing and Man-Thing, where the characters launch so closely together it's impossible for them to have been inspired by the other.
But then there are other times where the creator very intentionally chose to take a character everyone knew and alter it slightly to give everyone something new. Sometimes it creates a character that actually catches fire, and other times the creator just wanted to use someone similar to tell a story they couldn't tell elsewhere. With DC, typically as the older company, they aren't pulling ideas from Marvel very often, but it still happens more than one might think.
10 The Guardian
It only takes a single glance to recognize The Guardian as a Captain America analog. A character from the early '40s, Jim Harper was a police officer who worked in Metropolis and turned to being a superhero to stop criminals. He wields a shield like Captain America and doesn’t have powers, he’s just got great martial arts and boxing skills. Specifically developed by Captain America’s creators Joe Simon and Jack Kirby, The Guardian was introduced a year after Captain America made his first appearance.
9 The Terrifics
The main concept of The Terrifics is about a group of superheroes thrown together by chance and forced to work alongside one another. In other words, these characters weren't actually supposed to be a reference to the Fantastic Four, but they turned into it thanks to the ongoing in question. Mr. Terrific takes on Mr. Fantastic’s role, Phantom Girl can literally make herself invisible, Metamorpho is the Thing, and Plastic Man is the new Human Torch.
The series filled in a unique hole in the superhero world, as at the time the Fantastic Four weren’t around (presumably because they weren't able to be in the MCU yet) and the Terrifics provided high-concept super-science adventures.
8 Mongul
The long-time villain of Superman, Mongul was created by writer Len Wein and artist Jim Starlin. Starlin had created Thanos seven years prior, and when Len Wein wanted a character capable of physically challenging Superman, Mongul was what came out. Mongul runs his own alien planet and is constantly pushing for control over the universe. Much like Thanos, Mongul aims to bend the rest of creation to his will.
7 Mogo The Living Planet
Back in 1966, the character Ego the Living Planet was introduced to the Marvel Universe. The character was introduced as a villain and would make various appearances in Marvel’s cosmic comics over the decades.
Then in 1985, a character with a very similar name was introduced in Mogo. Though Mogo doesn’t talk, he’s a sentient planet that is responsible for protecting the Green Lantern Corps and sending out their rings to other potential Lanterns. The character has done everything from going into dormancy after losing his powers to being destroyed to being brought back via retcon at this point and is much more of a central character to the Green Lanterns than Ego is to Marvel.
6 Terra
Terra wasn’t specifically meant to be Kitty Pryde. But when both X-Men and New Teen Titans were on the market, they were competitors. Marv Wolfman took advantage of everyone’s expectations of Kitty Pryde and how nice she was to create Terra. The readers believed Terra would be like Kitty Pryde, but she would turn traitor over time and it would be a plot twist the readers would never expect.
5 Red Rocket
Red Rocket was an idea that Russia had to come up with an answer for America’s constant creation of superheroes. Though they couldn’t directly create superheroes, what they could do was outfit loyal members of their country with special armor that granted them super strength and advanced weaponry. This bit of technology makes them very similar to Marvel’s Crimson Dynamo villain.
4 Sam Lane
Sam Lane found himself retconned over time by writer Geoff Johns into a character that very much resembles Hulk’s Thunderbolt Ross character. A military man who’s obsessed with a super-powered being that happens to be dating their daughter.
In Thunderbolt Ross’ case, it’s the Hulk and his relationship with Betty Ross. Meanwhile, with Sam Lane it’s Lois Lane dating Superman, who Sam doesn’t believe has America’s best interests at heart. The character becomes a straight villain during the New Krypton arc, sending threats to attack the Kryptonians.
3 Steel
Steel is literally Tony Stark with a Superman twist. John Henry Irons was once a super-genius that made his money off designing weapons for different companies. But once he realized his weapons were falling into the hands of the wrong people, he wanted to make up for what he had done. After watching Superman, he was inspired to build a suit of armor to become a hero himself and help as many people as possible while also balancing out for the harm his weapons had done.
The main thing separating Steel from Iron Man is the character doesn’t create as many different suits as Tony Stark does.
2 Manhunter
Manhunter is what happens if someone takes Daredevil and makes them into a mom. Kate Spencer balances her life as a vigilante with her life as a lawyer. They have similar color costumes and Kate uses a staff while Daredevil uses a baton. Neither character is capable of balancing their lives very well, with Kate having ruined her marriage and barely holding on to her kid because of her devotion to work.
1 Tobias Whale
Tobias Whale is Kingpin with a gimmick of him looking like a creature from the sea. The character controls Suicide Slums in Metropolis and no one ever does anything about it, in the same way that no one ever does anything about the Kingpin. They do battle with heroic characters who have powers and always narrowly manage to avoid being charged with any crimes thanks to the power they wield in the criminal underworld.
In both cases they're being overlooked by much more powerful heroes and instead face heroes with more street-level abilities: with Kingpin his primary nemesis are Daredevil and Spider-Man, while with Tobias Whale it’s Black Lightning.