Although several sources (e.g. al-Biruni) appear to have considered 'Arash' to be the origin of the name 'Arshak' (i.e. Arsaces), the name of the Parthian dynasty derives from a Parthian- or Eastern Iranian equivalent of 'Ardashir', i.e. 'Artaxerxes', specifically Artaxerxes II, who the Arsacids claimed to descend from.
Arshak II was named in honor of his Parthian, Pontian and Armenian ancestors who ruled with this name as King, in particular he was named in honor of Arshak I, also known as Arsaces I, the founder of the Arsacid Parthian dynasty and the first ruler of the Parthian Empire.
Arshak III was named in honor of his late paternal grandfather; his Parthian, Pontian and Armenian ancestors who ruled with this name as King.
Towards the end of Antiochus II's reign, various provinces simultaneously asserted their independence, such as Bactria under Diodotus, Parthia under Arsaces, and Cappadocia under Ariarathes III.
•
Soon after however, a Parthian tribal chief called Arsaces invaded the Parthian territory around 238 BC to form the Arsacid Dynasty — the starting point of the powerful Parthian Empire.
He was known to be a great general who had many victories against the Persian king Shapur II, but was finally captured along with Arsaces II and flayed, his body displayed at the gates of the castle where Arsaces II was kept captive.