AI Civ having another civ's specialty unit
Solution 1:
In Gods and Kings, Militaristic City-States can sometimes grant the Unique Units of civilizations that are not in the current game as their unit gifts. Each Militaristic City-State grants a different UU, which can be found from that City-State's interaction screen.
Likely, the AI Civ in question made friends/allies with a nearby Militaristic CS, which is now granting them Cho-Ko-Nu.