Well, for one thing, there is the pain of the contractions, the breathing, the straining pushing the newborn, and the pressure on the vulva. And the experience lasts for hours depending of the woman or mare with ponies.



While the case with the anus is mostly short and sharp. While one is pushing a baby out, it's mostly inserting something in... or repetitively. Of course, the sphincter is not as flexible as the vulva, but that won't stop not only females to experiment with something a bit... maybe too large for them. Read dildo.



But the anatomy of a pony is not the same as the human anyway.