“A Case Study of Preferential Bestiality (Zoophilia)”, 2002 (; backlinks):
Humans show a wide array of sexual preferences and behaviors. Although most humans prefer and have sex with consenting adults of the opposite sex, some individuals have unconventional preferences with regard to the sex or age of sexual partners, or with regard to the nature of sexual activities.
In this paper, we describe a rare case of preferential bestiality, or zoophilia. The client meets the most stringent criteria for the diagnosis of zoophilia.
In particular, his phallometrically measured arousal pattern shows a sexual preference for horses over other species, including humans.
…In each offence, the target was a horse. He had no other convictions. The subject was the oldest in a family of 7 children. He grew up on a farm, and at an early age was given responsibility for the care and feeding of the farm animals. He reported that he was unaware of when his sexual attraction toward animals developed. However, he stated that he engaged in a wide variety of sexual acts, starting with chickens and finally settling on mares. He also reported that his involvement with horses was not limited to sexual acts, but also included a strong emotional component. In his most recent offence, he inserted his arm to its full length into the vagina of a mare and punctured its vaginal wall. The horse subsequently died. The subject reported that the mare had shown an interest in a stallion, and he had killed the mare as a result of jealousy
…This greater arousal was specific to horses, because the subject did not show arousal to pictures of other nonhuman species. As far as we know, this is the first time this has been demonstrated in the sexological literature.