Actually, these kind of weird gender things happen quite frequently. The cat mentioned above may have been intersexed, although they're not usually able to reproduce.
We had a young cockerel who was growing long black tail feathers, shiny pointy neck feathers, think legs, heavy body, and mounting the girls. One day, his tail feathers began to fall out and his appearance started to change. Soon, he stopped mounting the girls. In a few months, s/he actually started laying eggs. Now, she is just a very big and heavy hen.
Chickens have relatively simple sex organs, and with viruses or other kinds of damage, they can change. Usually, it is hens becoming roosters, but it does happen the other way around. Once my chicken finally dies, hopefully a long time from now, I am definitely going to have a necropsy done to find out what's going on inside.