While not technically meta-humans (as they are not, generally, even human), there are a plethora of conspiracy theories which believe that Earth has been visited by both extraterrestial and extradimensional beings.
There are many meta-human researchers who believe that there are extraterrestials living within the solar system:
No such civilizations have ever been confirmed to exist.
In addition to extraterrestials, some believe that there are beings which have come to Earth from dimensions other than our own:
Currently, none of these (or other) dimensions have been proven to exist.