I don't know much about the Atlantean people so I gotta ask you: Do mutants happen among them? I know Namor is a mutant thanks to him being half human but well some mutants aren't part of humankind like there's a mutant Brood and other wimsies.
Yes, mutations happen among the Atlanteans/undersea people, not necessarily X-Gene mutations but it could be a number of factors, often times we get mutated monsters due to pollution/chemicals, etc. However X-Gene mutants are among Atlanteans, Namor is a mutant due to his X-Gene that he got through his father's side which gives him his ankle wings/power of flight. Namor is the only case in which he predates the notion of X-Gene mutants since he was published long before the X-Men, a recap of a thread I once did:
Namor is the prototype mutant. Namor is the First Mutant because of Sub-Mariner comics issue 38. In Feb. 1955 Bill Everett (Namor's creator) wrote a comic called "Wings on his Feet" which explained why Namor has his ankle wings. Many people see Namor being a mutant as a retcon. I choose to see it as embellishing on already established canon. Everett wrote that when Namor was 14 he & his mother were in a dangerous situation & Namor manifested his wings as a way to save her and himself from being trapped in the ice., now remember this was in Golden Age comics, while the term mutants was around for horror this exact case of Marvel X-Men mutants, x-gene, teens and kids manifesting their powers through puberty or in traumatic times would come later in the Silver Age.
Uncanny X-Men (1963) is first published in September 1963, that is about 8 years after "Wings on his Feet" was published. A lot of what Everett established sounds similar to what Stan Lee & Jack Kirby did for the X-Men, (teens and kids manifesting their powers through puberty or in traumatic times) however now they had the X-Gene. Another thing is how the X-Men were always considered outsiders, just as Namor had been since his creation. I don't have hard evidence that Lee reused Everett's idea without crediting him but I have my theory that he heavily based the entire mutant powers manifesting concept on Everett's Namor as it was well known that Stan Lee loved the character of Namor a lot.
Also another time in which Namor is called a mutant before the X-Men's creation is Fantastic Four (1961) Annual 1 published July 1963, that is still a couple of months before Uncanny X-Men (in September 1963)
Here Namor is called the first known mutant:
Again even if you don't count the 1955 story, we still have Namor proclaimed as the first known (and published) mutant in Marvel Comics continuity; Hence why one of Namor's publication titles is "Namor: The First Mutant" and why Apocalypse is considered the in story canon "first mutant" but Namor is actually the first created mutant character, which is why you often see "First Mutant" on his covers.
Here on one of his 90s era covers it reads "Marvel's first and mightiest mutant"
A comic that I LOVE, also showcases Mutants with the X-Gene in Atlantean society: Dark Reign: The Cabal - The Judgement of Namor
Namor is called upon to a custody hearing between two divorced Atlanteans, while the mother hid her son's new mutantion, the father wished to use his son's new powers as a weapon to aid Atlantis. Namor is disgusted by both parents for neglecting the care of the child and takes custody for him as his ward.
The kid isn't happy about it and tries to fight against the ruling but Namor makes it clear he will be going to the X-Men's school.
A most recent and profound change or mutation in a character that doesn't have anything to do with the X-Gene is Bloodtide, pollutions in the ocean from humans changed her. Older Bloodtide look in left pic to the newer look in right pic, Avengers (2018) #10:
There are other characters like TigerShark who was mutated due to experiments with Namor’s genes/blood and those of a tiger shark, and of course others who have mutated due to one reason or another, via magical, science, or other means either natural mutation or a forced one.
Mutations of different kinds not related to the X-Gene definitely happens among Atlantean and other sea folk.
