My guess? Is that few remaining eternals that r still living on earth bear their powers to human babies, and their children grows up to obtained powers. And that's when normal people bring out the word to them,"MUTANTS".

No. Mutants are homo sapiens that have been introduced to the X gene. Eternals and Mutants however do share the same origin from Celestial experimentation

X-gene is what normal scientist call it when they find it in human dna.

@dewboy01: and 99.9% of the time when eternals have kids with humans the offspring are humans