Officially: Ghouls are a mix of FEV and radiation. Harold is a mutant who resembles a ghoul. Harold may have had some radiation damage before he was exposed to FEV, but his mutation (outside of his surface appearance) makes him different than a ghoul. Most ghouls are the result of extreme radiation + FEV exposure, but Harold's change was due primarily to FEV exposure (again, he could have sucked up a few rads in the wastes without him knowing, especially considering how long he ran caravans in the wastes).