A kingdom of iran was known as Partha. You know how many of Hindu Gods' names have been derived, it is sometimes from people who became legends. Example- Indra - The leader who destroyed many Dravidic forts during the Vedic period. Another is Maninder, which seems to have crept in from a bactrian prince's name.
My guess is that a king of Partha came to be known as Partha, and subsequently it became a divine name