Here's a bit more clarification
it had illustrations and everything
The woman was at a ranch and was drugged through the beginning as she signed waivers. Originally got there seeking a job
The rectum was lined up with her body so the shit went through her and out her mouth
then after some time she was impregnated by the people working at the ranch
Some keywords from the 4chan post that linked it were something like "this happens in the same universe as the human centipede, I see" (that was close to the exact quote but definitely universe, human, and centipede were in the post), I believe it was on /lit/ or similar board. Perhaps /tv/ or /int/
she breathes out the anus like in the OP and is absorbed over time, her skin fuses with it and is able to survive as one with the horse
Here is an approximate picture of the offspring
