When were the Comitatenses formed? | The Birth of the Late Roman Army 

The Late Roman Army was divided into tow broad groups: the Comitatenses, or Field Armies, and the Limitanei, or Frontier Armies. The first oficial mention we have of the Comitatenses is in 325, when documents explictly refer to it to distinguish it from the frontier forces under the Emperor Constantine. However, military experiments under the Emperor gallienus in the third century, constructing an army made of cavalry, and further military refomrs under the Empero Diocletian, have led some schlars to wonder if the creation of Late Roman field armies can, or even should, be pushed back past 325 to the third century proper.
SOURCES:
The Late Roman Army, Southern & Dixon
The Roman Army: A Social & Institutional Hisoty, Southern
The Roman Army, Southern
Roman Warfare, Goldsworthy
Warfare in Roman Europe, Elton