A little research ...

The origins of the Parthian people are clouded. Strabo (xi, 515) says the first Arsaces was a Scythian man with the semi-nomadic Parni tribe, a part of the Dahi, nomads who lived along the Ochus (Tejend or lower Oxus) River, who invaded and conquered Parthia. Strabo also mentions those who claim Arsaces was a Bactrian who escaped from Diodotus after a failed revolt. Justin (xli, 1) agrees Arsaces was a Scythian. Frye's analysis is that we can believe the Parni origins, but it was more likely a migration than an invasion that brought them, and Arsaces, to Parthia. (History, p. 207) These people would not be known as Parthians until they moved southward into the Persian province of Parthava sometime before 250 B.C. Achaemenian and early Greek references to "Parthians" refer to earlier inhabitants of Parthava, not the Arsacid Parthians. (Debevoise, Political History, 2; W. M. Montgomery, Early Empires).

hhhmmm