I can't think of any example of such soldiers in the Republican or early Imperial Roman army. But in the sixth century Procopius describes Belisarius and Narses, whose armies were made up signifigantly of cavalry, dismounting cavalry when they needed infantry on the battlefield, and forming these dismounted horsemen into a "phalanx." Since a large number of soldiers in the army of the period were recruited from Germans, Slavs, and steppe people, this was tactic was probably adopted from the "barbarians."