Half-human, half-monster boys. Though they can take on similar species to that of monster girls.
(Keep in mind, some monster girl species such as harpies or lamias can have male examples, as they might be built on the same concept but have no male equivalent word.)
Do not tag animal-eared boys, such as cat boys or rabbit boys, or specific types like demon boy. Do not confuse with the monster tag.
The following tags implicate this tag:
harpy_boy, lamia_boy, merman, plant_boy, and slime_boy