A mutant is a kind of sentient person on Boop that is defined by being a non-naturally occurring organism.
Monsters are differentiated from monsters due to the fact that monsters are born, while mutants are made.
A mutant was once, either in its life or several generations back, a different kind of being. Plants, birds, fish; all once normal and non-sentient are changed by a mysterious process to become fully intelligent humanoids.
These mutants are not easily defined, and primarily are considered mutants if they are not a kind of "classical monster". A dragon which randomly hatches from a normal lizard egg might be considered a monster, but a humanoid lizard hatched from the same brood would be called a mutant.