Guy Monsters are what we call the Male creatures of the third Monster generation. They are a part of the Planner Scenario, designed to be fodder enemies for humanity. Along with the Gal Monsters, they were originally born from the Four Holy Gal Monsters, the mothers of all monsters.
While they're all technically the same species, they vary massively in terms of size, shape and general structure, ranging from formless to humanoid. Usually, they mature in less than a year and can live up to 100 years, many of their species are edible, resulting in them being hunted down for the creation of delicious meals.
They typically breed with Gal Monsters, the monster-type and size differences do not matter as their genitals have been designed to guarantee successful impregnation.
Guy Monsters come in many different shapes and sizes, but all of these creatures fit into unique categories that allow one to classify them.