Some of the monsters that King Piccolo had spawned long ago still survive on Earth. Some were secretly incubated in places such as Pilaf's castle, and others that had bred naturally in the wild. Piccolo became aware of their existence, but determined he could not resolve the situation on his own, and so he accepted it.
A while after the Namekians had emigrated to Earth, at Great Elder Moori's suggestion, Piccolo and a few other Namekians capture these monsters and keep them secured. In the midst of this, some Dragon clansmen started to research the monsters and were able to master handling them. This knowledge leads to the birth of "Poko Priest" class.