The source is obviously treating zoophilia in the context of forensics and law so it has some uglier bits, but it's an interesting foundation for more dialogue on the topic of definitions.

1.1.1. Class I zoosexuals: human-animal role-players

Class I zoosexuals do not use animals for sex at all, just as class I necrophiles do not have sex with dead people. In other words, they are mere role-players. They would rather want their human partners to act as animals during sex, because the thought of having sex with animals excites them. Also known as pet play, pony play, ponyism or pup-play, humaneanimal role-play thus involves one participant taking on the role of a real or imaginary animal in character, including appropriate mannerisms and behavior. Thus it is a kind of sexual role-play.

The principal theme of human-animal role-play is usually the voluntary or involuntary reduction (or transformation) of a human being to animal status, and focus on the altered mind-space created. The most common examples of “conversion” are canids (pup, dog, wolf), felines (cat, kitten, lion) and equines (pony, horse). The activity is common among people with a transformation fetish (A form of sexual fetishism where a person becomes sexually aroused by descriptions or depictions of transformations, usually the transformations of people into other beings, animals or objects). Human-animal role-play is also used in a BDSM context, where the partner is reduced to the status of an animal as a symbol of showing authority and dominance.

1.1.2. Class II zoosexuals: romantic zoophiles

Would keep an animal as a pet in order to get a psychosexual stimulation. Would not actually indulge in sex with the animals. Ebing’s zoophilia erotica fetischistica would fall under this class.

1.1.3. Class III zoosexuals: people having a zoophilic fantasy - zoophilic fantasizers

These people fantasize intercourse with animals, but do not indulge in actual intercourse. May masturbate in the presence of animals. Zoophilic voyeurism (also called mixoscopic zoophilia or faunoiphilia) and zoophilic exhibitionism would fall under this category.

1.1.4. Class IV zoosexuals: tactile zoophiles Interest in animals increases to the level of touching them.

These are the people, who get sexual excitement by touching, stroking or fondling an animal or their erotic parts, such as genitals or anal and perianal area. Some would rub their genitals against animals, as a source of pleasure (zoophilic frotteurism).

1.1.5. Class V zoosexuals: people having a fetishistic zoophilia e fetishistic zoophiles

People belonging to this class may be called animal fetishists.

They preserve parts of animals, especially furs and use this as a fetish for their zoophilic activities. The touch of soft and silky fur of animals may act as an erotic stimulus, just as ordinary fur does for a fetishist. Other common fetish objects such as shoes would not sexually stimulate this group. The fetish object must be part of an animal. Randall and associates12 narrate an interesting case, where the tongue of a deer was used as a masturbatory tool.

1.1.6. Class VI zoosexuals: sadistic Bestials

Sexual pleasure comes from sadistic activities with an animal, such as torturing it (zoosadism, zoophilic sadism or bestialsadism).

People up to this class, use animals for sexual excitement, without actually engaging in intercourse with them. Bartmann and Wohlsein13 describe traumatic injuries in 193 horses over a four year period, out of which at least ten cases were due to acts of zoophilism and zoosadism. The injuries were caused by gunshots, or by spear like instruments or knives. Only seven could be saved by surgical or medical treatment. Wochner and Klosinski8 studied 1502 aggressive children and adolescents who had been presented and treated at the Department of Child and Adolescent Psychiatry at the University of Tübingen, and found that out of them 25 had been zoosadists. These were exclusively boys (23 out of 25). The age distribution of the zoosadists showed an increased incidence in 13, 17 and 18 year olds which could be connected with problems of puberty, group constraints and proving virility. Compared with a control group of “only aggressive” patients, organic brain damage owing to complications of pregnancy or delivery, difficult and harsh upbringing by the parents and absence of a positive father figure could be demonstrated in the zoosadists. One third of the zoosadists showed additional disorders of sexual behavior and the sexualsadistic component was manifested in the zoosadistic action.

1.1.7. Class VII zoosexuals: Opportunistic zoosexuals

Actual sexual activity with animals starts from this class. Normally these zoosexuals would be content to have sexual intercourse with the living[humans], but if an opportunity arose, would not refrain from having sexual intercourse with animals. Such activity may be seen in incarcerated or stranded persons, or when the individual sees an opportunity to have sex with an animal with no one around (e.g. a shepherd taking cattle away for grazing on a lonely farmland). Attendants of animal houses also belong to this class. People belonging to this class do not love animals at an emotional level.

1.1.8. Class VIII zoosexuals: regular zoosexuals

The so-called “classic” zoophiles. They do not enjoy sexual intercourse with humans and prefer animals for intercourse. They can however have sex with both humans and animals. In this sense they differ from class X zoophiles, who can have sex only with animals. This class has subclasses including activities such as fellatio, cunnilingus, masturbation of animals and anal intercourse with animals. These are the people, who love animals at an emotional level, and express their love through sexual intercourse.

They have sex with animals, as one would normally have with human partners. These people tend to love animals, and by and large do not hurt or harm animals.

A subclass exists within this class, which may be called “regular zoophilia by proxy”. In this case, the person e instead of himself having sexual intercourse with an animal - may force his or her partner to have a sexual act with an animal. This happened in R v Bourne (1952) 36 Cr App R 125 (CCA), in which the husband forced his wife to submit to a dog inserting its penis into her vagina. The husband was convicted of aiding and abetting his wife to commit buggery and sentenced to eight years in prison. The wife was not punished, since she acted under duress. In R v. Tierney (1990) 12 Cr. App. R.(S) 216, the defendant took photographs of his wife having intercourse with his Alsatian dog for his own continuing satisfaction.

In this case, three months’ imprisonment was given to the accused, but not to his wife, because she consented to perform the act in desperation in order to retain her husband’s affections.

1.1.9. Class IX zoosexuals: homicidal bestials

These zoophiles need to kill an animal in order to have intercourse with it (necrozoophilics). They are however capable of having sexual intercourse with living animals, but the need for sexual intercourse with the dead animals is so great that they must kill animals in order to have sexual intercourse with their dead bodies.

The noted killer Jeffrey Dahmer (1960e1994) is reported to have collected animal carrion from road, dissected them and masturbating, as he found the glistening viscera of animals sexually arousing.14 In November 2006, Bryan James Hathaway, 20, of Douglas County, Minnesota was arrested for having sex with a deer carcass he found on the side of the road while bicycling. He was charged with violating a law against “sexual gratification with an animal.” He was placed on probation, and was also sentenced to be evaluated as a sex offender and treated at the Institute for Psychological and Sexual Health in Duluth.

1.1.10. Class X zoosexuals: exclusive zoosexuals

Sexual intercourse is possible only or mostly with animals, with virtually a complete exclusion of human partners. This group has been called zooerasts by some writers.

[Source Abstract]