The first set of hidden Monokumas can appear after free time opens up in Chapter One. The earliest you may find one is on the hyacinth terrace southeast of the dormitory, as it doesn't require entering any additional rooms (that might happen frequently if trying for these when you're restricted to certain story elements).
For the sake of giving you variety, I shall at least list the first four available during the free roam (Free Time) opportunity given to you in the first chapter, in no particular order:
*** Spoiler - click to reveal ****: Outside the dormitory diagonally southeast to the hyacinth terrace.
*: Heading from the dining room down the hall to the gym, you will come across a small dining table row with the Monokuma found among them.
*: On a sink in the girls' bathroom.
*: Among the machines in the game room of the basement.