Ancient Mercenaries were fairly common and varied greatly in terms of quality. The easiest example to draw upon here is the Carthaginians. They often used a great deal of mercenaries as Carthage was more concerned with naval capabilities and economic power, whereas compared to Rome which had a culture and heritage of war.

The most notorious of the Carthaginian mercenaries were of course the Numidians. Numidians were a north African semi-nomadic people who boasted excellent cavalry, arguably some of the best mobile cavalry at the time. They prided them selves on hit and run tactics, the feared feigned retreat, raiding, patrolling, and scouting. Numidian horsemen were used effectively by Hannibal Barca during the Second Punic war. In fact many historians attribute some of his victories solely to his excellent cavalry arm. Hannibal was incredibly sucessful and used his Numidians to great effect - even catching the Roman Proconsul Marcus Claudius Marcellus in 209 BCE and killed him, which obviously dealt a significant blow to Rome - which already greatly feared Hannibal's horsemen. (much of this can be found in Polybius' "The Histories", as well as the chapters dedicated to Marcellus in Plutarch's Lives Volume one.

Keeping in the same vain - not all mercenaries were created equal, while the Numidians are perhaps some of the most prolific mercenaries to take a battlefield (Rome would even later purchase (rent?) some as Polybius tells us). Many of the foot infantry in Hannibal's army were not the excellent grizzled Iberian and African infantry - but barbarians, Gauls and Iberians. You see, Hannibal had a policy of convincing cities, tribes and peoples to rebel against Rome and join his cause (and army), of course many of these people were barbarians, which needed little convincing as they harbored a resentment for Rome. He would promise great riches to these irregulars as well as revenge of course. Most of these Barbarian mercenaries were footman as Polybius writes, but there was some cavalry which was also rather irregular. These troops were often inferior to the Roman infantry and the Numidian horsemen, they used worse weaponry and little armor, often lacked advanced formations your might see in a Roman legion. However, Hannibal had no choice and they did serve a valuable purpose in that he was able to march a great distance and recruit more troops in the invaded territory, they were critical to his success even if they were unreliable.

I'd recommend reading Polybius in particular for more on this, I hesitate to recommend Livy as he is frankly unreliable. If you're curious for more specifics I can offer those later - I don't have many of my books with me currently. I should also add, that a big reason the Numidians were so successful against the Romans is that the Romans did not yet have a good understanding of cavalry nor good cavalry in general, Hannibal's cavalry would frequently defeat Rome's. I believe the Numidians defeated cavalry at Cannae and then struck from behind the roman formation to seal the victory.