"Original" Croats were Slavs. Their personal names, language and religion was Slavic since they were first time mentioned in the sources. The places called after Croatian etnhnonym were found in the areas where the historical sources mention Slavs or where the first slavic states emerged, Serbia included.
Croatian ethnonym itself is, however, most probably of Iranian origin, but the slavic word for "god" is as well. All that strongly support the Slavic origin of Croats (or vice versa, who knows). The proposed area of the Croatian origin fits pretty well with the area of highest diversity of the I-CTS10228 subclade, as well as with the area of the earliest Slavic toponymy. Present day Croats are descendants of these Croats and autochtonous population of Panonia and Dalmatia who were a minority at the time of Croatian arrival.
Names of Croatian leaders from VII century were Kluk, Muhol, Kosen, Lovel, Tuga and Vuga and that names are not Slavic.