She (now sounding rather furious): "Why don't you want to let my mare become covered by your black stallion?"
Me: "Because he isn't a stallion anymore!"
She: "What do you mean with that?"
Me: "My boy is a gelding."
She: "So what? I want to have a foal from him."
Sycorax
"That was the moment my jaw dropped ..."
This person should never have been allowed to buy a horse, let alone breed them

. I worked at a barn in high school, and we gave trail rides and had weeklong horse camps for kids during the summer. I only remember one "not gonna happen" request though.
Me: You'll be riding this horse, her name is Daisy.
Guest: ::face falls:: Do you have a white horse? (Daisy is an Appaloosa, a spotted breed of horse)
Me: Um...just one, but he's very old and for his wellbeing we only have children ride him.
Guest: What about that horse? ::points to nearby run where a border's stallion is standing:: (Stallion is a paint, but has few markings and from that angle he looked all of one color)
Me: The stable doesn't own that horse, and besides, he's a stallion. Only very experienced riders ride him.
Guest: Why can't I ride him?
Me: He doesn't belong to us, and he's dangerous. (he wasn't really, but would be for an inexperienced rider)
Guest: But...why can't I ride him?
Me: Because he'd hurt you.
Guest: Oh. ::I help guest onto her horse, and then bring my horse out from inside the barn. My horse at the time was very light fleabitten grey, i.e., looked white) Can I ride that horse?
Me: This is my horse, and she needs an experienced rider.
Guest: Can I ride her just a bit at the end of the ride?
Me: ::wishes Daisy would buck guest off and stomp on her::
I have no idea what the fixation with "white" horses was. It was weird. Working at the bookstore for awhile, I also had a number of times where a patron could not understand that we did not have a book in stock. No, it was not in "the back". No, looking in "the back" multiple times would not make it magically emerge.