I recall a few zoo stories in the Penthouse Letters magazines. Most were absolute zoo fantasy trash of the poorest kind. One really bad one that stands out in my mind was Black Zoo Girl. It was a story of a trailer park slut who fucked all the guys and animals in her trailer park, including a bear that was raiding the garbage cans.
Funny how every sexual encounter involved her sitting on the edge of something with her legs spread and the man/dog/goat/pony/bear understanding the invitation and instantly having sex with her in that position. The fact that for animals it would take a lot of time to get them to mate in any position besides their instinctive "doggy style" position.
All were completely forgettable with a single hint of the writers knowing how zoo encounters really take place or the biology of any of the animals in the story. Zoo porn stories written by virgins for virgins is my guess. Especially since all the other letters to Penthouse were fiction written by virgins for virgins.