The only "real" heartless, ones with absolutely no markings are shadow, Neo heartless (notice how they kind of look like disfigured people with antennae) darkballs and the bat thingys when you fight ansem. The invisibles have the heart shape cut out of them same with the darkside heartless. I consider that a marking but whatever. So he could have turned into a Neo or darkball, but Neos didn’t exist yet and darkballs are...uhh, weird? He could have turned into an artificial heartless if one stole his heart, like the man in traverse town, second district.
EDIT: The pot and barrel spiders dont have a heartless marking on the outside, but who knows whats on the inside?