The history of zoophilia and bestiality begins in the prehistoric era, where depictions of humans and non-human animals in a sexual context appear infrequently in European rock art.[1] Bestiality remained a theme in mythology and folklore through the classical period and into the Middle Ages (e.g. the Greek myth of Leda and the Swan)[2] and several ancient authors purported to document it as a regular, accepted practice—albeit usually in "other" cultures.
Source: https://en.wikipedia.org/wiki/History_of_zoophilia