Yes this is a Croatian belt. Yes it is original. But IS NOT Ustasa.
This belt was used by Army (Domobran) officers from Lieutenant to Colonel. The same ranks for Navy (Ratna Monarica) and Air Force (Zdrakoplov).
Below are some pictures with the difference between your belt and an Ustasa belt. This is from my collection. I have extra Ustasa belts if you need one.