I thought it was rhinos, but who knows if it's true...?
The explanation given is that manoevering the relevant part by moving the whole body about (as cattle, horses etc do) is too difficult with such a large animal.
The end therefore is able to "walk" it's way to the other relevant part.
One wonders if the female appreciates her partner's extraordinary capabilities -- if they do in fact exist in either group.
I recall seeing film of mating rhinos. Motility of the part was not apparent in this, but I remember that the male had to perch his whole body on top of the female, no feet touching the ground. He then fell off, sideways, all two or three tons of him, and later wandered off with an injured shoulder. I don't think he saw the joke...