Parthian shot is one of the most contensious issues in the whole of wargames.
It killed WHaB for many players when 'Nomad horde' armies came to the fore.
It doesn't happen in DBMM, as Light horse don't shoot, and only flee certain elements.
What you need is a 'feigned flight' ablility, where your units, when charged can shoot and then retire, but should need to take some kind of skill test to avoid getting caught; or they need a half moove/fire/second half of move ability. The WHaB problem was the Light horse had (effectively) a double move and shot move, meaning they could never be caught...
Historic armies that had to deal with Partians were:
Romans (Marian & Early Imperial),
Sassanid Persians (effectively a civil war as the Parthian dynasty was overthrown in internal revolt)
Later Selucid Empire (The Parthians were originally an independence movement from Greek rule)
Skythians (or other Nomad hordes) and other Western Indian States.
Honestly, one of the best places to look for historical opponents is the old DBA v1 rules, which gave you a list of historical opponents. Unfortunately mine are lost in the attic!http://en.wikipedia.org/wiki/Parthian_Empirehttp://www.livius.org/pan-paz/parthia/parthia02.html
Hope that helps