"Monster" is an individual who has a profoundly different structure (due to abnormality or deformity) than that of other individuals of his own species.
In ancient times, above all in the Greek one and even more in the Middle Ages, those who were not subject to the laws of nature, deriving sometimes from couplings between different animals, or between demons and animals or between men, were considered "monsters". Therefore the "monsters" have always been considered as a subject of horror. However, in the various historical epochs, acute observers who considered these phenomena as completely natural were not lacking. Aristotle, Cicero and Pliny, were among the first to possess that spirit of natural observation, which in Italy was increasingly affirmed in Humanism and the Renaissance, in which naturalistic concepts were the basis of the interpretation of monstrous phenomena.
So long and difficult was the path that the word "monster" had to face until Isidore Geoffroy Saint-Hilaire, in the last century, was able to clarify that even the so-called monsters are not supernatural or invention beings, but also subject beings they to the laws of nature. Even today its classification for these particular beings is useful.
Thus the branch of the embryology branch is born, which aims to study the monstrosities or morphological anomalies of the whole individual or part of it. Currently the teratological phenomenon refers to the result of an abnormal ontogenesis, in one of its stages. There are frequent anomalies and cases of monstrosities that occur among domestic animals. According to the morphological classification of Taruffi, there are unitary monstrosities (Terata monosomata, Tar) and multiple or compound monstrosities (Terata polysomata, Tar.).
Among the very common unitary monstrosities among the domestic mammals are those relative to some parts of the individual (mono-merosomata Terata, Tar.), As the anomaly of the skull, and that of the face. The macrocephaly, anomaly of the skull, is recognized by an abnormal volume of the skull, very often associated with an imperfect development of the brain and sometimes with a modification of the shape of the skull itself: oocephaly (egg-shaped skull).