Were they of Croatian or Serbian origin it is unknown…
It is known, one just has to read, rather than to imagine.
Of the Pagani, also called Arentani, and of the country they now dwell in.
The country in which the Pagani now dwell was also previously possessed by the Romani whom the emperor Diocletian translated from Rome and settled in Dalmatia. These same Pagani are descended from the unbaptized Serbs, of the time of that prince who claimed the protection of the emperor Heraclius. This country also was enslaved by the Avars and made desolate and repopulated in the time of Heraclius the emperor. The Pagani are so called because they did not accept baptism at the time when all the Serbs were baptized. For 'Pagani' in the tongue of the Slavs means 'unbaptized', but in the tongue of the Romans their country is called Arenta, and so they themselves are called Arentani by these same Romans.
In Pagania are the inhabited cities of Mokron, Beroullia, Ostrok and Slavinetza. Also, they possess these islands: the large island of Kourkra or Ki[size=1pt]'[/size]ker, on which there is a city; another large island, Meleta, or Malozeatai, which St. Luke mentions in the 'Acts of the Apostles' by the name of Melite, in which a viper fastened upon St. Paul by his finger, and St. Paul burnt it up in the fire; another large island, Phara; another large island, Bratzis. There are other islands not in the possession of these same Pagani: the island of Choara, the island of Ies, the island of Lastobon.
– De Administrando Imperio chapter 36, Constantine VII, 950 AD (link)