Legio X Gemina, the twin legion, was levied by Julius Caesar on 58 BC, for his invasion of Gaul. There are still records of the X Gemina in Vienna in the beginning of the 5th century AD. The legion symbol is a bull. Early on its history, the legion was called X Equestris (knights), because Caesar often used the legionaries as cavalry.
In the Gallic wars, X Equestris played an important role on Caesar's military success and for this reason is sometimes said to be his favourite. In Caesar's campaigns they were present in the battle against the Nervians, the Roman invasion of Britain, and the siege of Gergovia. They remained faithfull to Caesar in the civil against Pompey, being present in the battles of Pharsalus (49 BC) and Munda (45 BC). In 45 BC Caesar disbanded the legion, giving the veterans farmlands near Narbonne.
The legion was reconstituted in 42 BC and fought for Augustus, Lepidus and Marcus Antonius in the Battle of Philippi against the murderers of Caesar. After this, they followed Marcus Antonius in his campaign against Parthia and were defeated with him at Actium. Augustus then took control of the legion and settled the veterans in Patras. The legion rebelled and lost the title of Equestris as punishment. Reinforcements were added from other legions and the tenth was rebaptized as Gemina.
The newly formed X Gemina was relocated to Hispania Tarraconensis, were Augustus was preparing a caimpaign against the Cantabrians. They stayed in Hispania for many years and their veterans were among the first habitants of modern Zaragoza.