The Exos are a sentient machine race that came about close to the Collapse.
Human sized machines with differing design elements.
The number in their name refers to the number of "Reboots" that they have undergone. The Reboots resets the Exo's memory.
The Exos started as self-aware war machines. Their exact origins are not commonly known. They are perceived to be so advanced that only a Ghost can understand their inner workings.