Danganronpa V3: Killing Harmony (EP)

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: