I am afraid I have never heard of a penis transplant, especially one from an
animal to a human (except in the movie Sex & Zen), so that is not
the explanation.
Warning: I am about to ruin many people's fantasies in the next paragraph.
Stop reading now if watching porn films with well-endowed men is your thing!
There is no such thing as an 18-inch long penis. Yes, I have seen some of the
same movies and they look quite authentic, but they are not. Have you ever seen
a science fiction movie where someone grows horns, or it looks like they are
being stretched, or better yet, an alien is walking around? Well, it is done
the same way, only slightly more low-tech.
There are quite a few special-effect prop makers who are only too happy to
moonlight on the set of a porn film. Those giant penises you see are
prosthetic. Yep, they are fake. Even the semen that comes out is
synthetic. Did you ever notice how the stars with REALLY large penises often
wear a sort of harness? That is because the artificial penis is so heavy, it
can't simply be glued on. Other times, it is glued on with synthetic skin
matched to the actor's skin tone, blended in to make it look seamless.
Anything over 11 inches is almost guaranteed to be a fake. Hope I didn't
destroy your fantasy.