Cicero frequently compares the monster to an animal (pig, dog, snake, wolf, bird of prey, predator...). The purpose of this article is to precise the possible links between animality and moral monstrousness. Does the figure of animal contribute to define the notion of moral monstrousness? Is the comparison able to distorse the notion of animality? It is possible to define two main kinds of animal figures: domestic, herbivorous and placid animals (livestock); aggressive, carnivorous and wild animals (predators). The comparison to livestock is used by Cicero to underline the monster's intellectual weakness, laziness, foundness for occupations considered unworthy of human beings. The monster's accomplices are compared to trained animals, through the herd and the dog figures. The comparison to predators is used to underline the monster's ferocity, the danger that he represents for human society. Indeed, Cicero makes feel that the monster can not be controled. The study of these different comparisons reveals the ambivalent logical link between animality and humanity: the animal may appear as a model, because animals are perfect examples of adaptability to nature, but humanity is constituted by the development of man's rational specificity. Even if the comparisons may be pertinent, they contribute to attenuate the idea of the moral neutrality of animals, distorting the notion of animality through an anthropomorphic vision. The comparison is thus based on a very superficial and conventionel vision of animal. The use of multiple animal figures to describe a monster may appear incoherent, but it permits to perceive the monstruous personality as an incoherent, unstable nature: the monster is a man who can be in the same time pecus and fera. So all the animal figures used by Cicero to describe the monster form a logical and coherent system: as regards the numerous and different breeds included in "animality", this notion appears as polymorphic and as difficult to define as monstrousness.