When Stan Marsh was banished from South Park when he refused to vote for the new mascot, The members of PETA found him while riding a horse and reluctantly take him, show a lot of care for the horse, and almost none for Stan, but they let Stan join, saving him from starvation in the wilderness.
The members love animals so much, they have become intimate with them, even to the point of bestiality and marriage.
Stan was taken to the leader of PETA named Dr. Cornwallis, who turned out to be a goat, to ask permission so he can live with the group, the goat bayed, and Stan said he was in, which the PETA members believed.
When Stan tells the leader of PETA why he was banished from South Park, the man actually gives a reasonable argument to Stan's decision by telling him that elections are always between undesirable candidates since those people have sucked up enough to get that far up in politics. It is ironic that this reasonable argument came from a group considered undesirable, when South Park didn't give Stan any reason to vote other than 'because you have to'.
P. Diddy eventually catches up with Stan again at the PETA camp and threatens him for not voting. When a group member throws blood on his fur coat, P. Diddy and his men take offense by killing all of the PETA members. Stanley is shot in the process, and remains the only surviving. The animals they defended and mated with, however, showed no interest in their deaths, some even roaming freely. After the massacre at the camp, Stanley goes back to South Park and votes. When one of the staff shows that the PETA Members were murdered, the students are initially shocked until Clyde announces that they can go back to being the South Park Cows, and cheers, much to Stan's annoyance.