Monsters arise from the boundary between this reality and the others. They are markers on the roadway that stretches from the mundane to the cosmic. Some roll and fall into the chasms of those foreign dimensions -- waiting for us to find them -- while others have been scattered about our world to bring tumult and love and misery and fear, because these things give them pleasure and sustain their material bodies.
Monsters change slowly or very quickly or never. Their occult metabolism is different than our steady growth and decline. It is a crooked sine-wave evolution from the serpent to the Dragon.
Most monsters hatch from eggs.
Maidens are often sacrificed to powerful monsters because their flesh tastes better than that of crones, and infants are tawdry nibblings compared to the succulent plumpness of a well-fed woman-child. Monsters are connoisseurs of mouth-feel.
Monsters conceal themselves from the light because in darkness there is protection from Reason. Light reveals their true forms and so denudes them of their monstrosity.