A Choboko is an overworld turn-based enemy. What seems to have the highest chance to spawn them is the area west of Papunika Village, in the area around the Hidden Meadow. I found it in about 3 encounters, but your mileage may vary it is a very rare encounter.
Not an achievement, but if you take the choboko meat you get as a drop, to the grandma in Aogai village she will cook the meat. If you give the cooked meat to the guy in the tent near the shrine of babamut he will give you an extra heart container. You will need airship