Your example seems like maybe independent invention....
Given that mainstream music for the last number of generations has been essentially played on the same instruments, and following the same musical theory, one must come to the conclusion that
everything has been played before.
Suggesting that someone plagiarized another's music without being able to show that they were exposed to the previous authors material is dicey business.
More likely is that
both Elvis and Wagner caught that riff from someone else and incorporated it into their own pieces.
