Docking
With docking to describe the insertion of human foreskin Penisses in the pocket (Felltasche) of the dog. This practice is more for the sexual satisfaction of humans since the dog's penis is not stimulated enough.
Docking may sometimes refer to the process of cropping a dog’s tail into a little nub, such as with the Rottweilers, Dobermans, Boxers, and others (in countries where the practice is still legal). It may also refer to the sex act in which a human penis is inserted into a male animal’s sheath.
Supposedly this can also be done with equines and other ungulates, however I feel that potential problems would have less to do with size accomodation and instead would be more related to the animal’s size and strength. If attempting this with their animal lover, zoos would be advised to go slowly and use a lot of lubricant.
Reference
https://web.archive.org/web/20070114032749/http://zoowiki.zetapin.de/index.php?title=Docking http://beastiality.club/beastiality-club-extreme-animal-sex-content/the-encyclopedia-of-zoophilia/