This article analyses Leonardo’s interest in monsters and deformed reality, one of the lesser known aspects of his vast and multifaceted output. With the possible exception of his studies of physiognomy, relevant drawings, sketches and short stories represent a marginal aspect of his work, but they are nevertheless significant for historians of teratology. The purpose of this study is to provide a broad overview of the relationship between Leonardo and both the literature on mythological monsters and the reports on monstrous births that he either read about or witnessed personally. While aspects of his appreciation and attention to beauty and the pursuit of perfection and good proportions are the elements most emphasised in Leonardo’s work, other no less interesting aspects related to deformity have been considered of marginal importance. My analysis will demonstrate that Leonardo approached the realm of monstrosity as if he considered abnormality a mirror of normality, deformity a mirror of harmony, and disease a mirror of health, as if to emphasise that, ultimately, it is the monster that gives the world the gift of normality. Two special cases of monstrosity are analysed: the famous monster of Ravenna, whose image was found among his papers, and a very rare case of parasitic conjoined twins (thoracopagus parasiticus) portrayed for the first time alive, probably in Florence, by Leonardo himself.
