My two cents, without claim of exclusivity.

Commagene. Their cataphracts gave the Roman armies a run for their money and had a majot effect on the Romans' later development of heavy cavalry of their own.

Pergamon (sp) Their heavy cavalry, led by Eumenes II played a major role at Magnesia, crushing the Seleukid left wing and helping the Romans win the battle. That alliance was crucial for the Roman establishment in Asia Minor and the famous Pergamon Heavy Horse influenced the Roman creation of heavy cavalry later on.

Numidians - the constant raids, rebellions and light skirmish tactics eventually forced the Romans to develop more mobile units in Africa and to expand beyond the territories of Carthage, in order to remove the Numidian Threat.