Who or what were the Costoboes?

[Epictetus]

During the reign of Antoninus Pius there were some disturbances in Greece by people called “Costoboes”.

An army of bandits, called the Costoboes, who overran Greece in my day, visited among other cities Elateia. Whereupon a certain Mnesibulus gathered round him a company of men and put to the sword many of the barbarians, but he himself fell in the fighting. This Mnesibulus won several prizes for running, among which were prizes for the foot-race, and for the double race with shield, at the two hundred and thirty-fifth Olympic festival. In Runner Street at Elateia there stands a bronze statue of Mnesibulus.

Pausanias, Description of Greece, 10.34.5

Does anyone have an idea who or what the Costoboes were? Pausanias calls them “barbarian,” so they must have come from somewhere outside of Greece, and were not an indigenous group of bandits.