"What? Who?"
The farmhand grinned and pointed towards Dan.
"What? No, I won't have sex with my-"
Suddenly, Dan fell silent. Kat could hear him starting to pant.
"Dan, what's wrong?"
Then she noticed the veins popping up all along his equine body. Layers of thick muscle started to develop, causing him to blow up in size. His arms and hooves became twice their normal size. And his cock...
Kat watched in horror as her brother's cock got harder and harder, his tip fattening up into an arrow pointed straight at her. And his balls-- they were practically hanging off his body.
Dan looked at Kat with fear and sadness in his eyes. "Kat...I'm so horny. I-I need to have you."
"Dan, stay back. Don't you dare put your penis in me." But Dan no longer cared. He grabbed Kat with his strong hands and pinned her arms back. With his hips, he flicked up his cock and entered her in one swift motion.
"Kat, you're so warm and tight. Keep squeezing my cock."
Kat tried to struggle, but as Dan's cock got deeper and deeper into her, she stopped fighting. Her vagina was naturally tightening around Dan's cock, while her womb was opening up. Kat realized that her body was already preparing her for pregnancy. She knew there was nothing else she could do but get it over with.
"Dan, I want you to cum. Shoot all of your nasty horse sperm into my womb!"
As eager as he was, Dan still tried to hold back. Kat then squeezed him tighter, until he could no longer take it. With a series of cries, he exploded, releasing waves upon waves of semen into her. Kat could feel it splashing around.
But at least it was done. So what if she would have her brother's seed? She'd just be giving birth to a bunch of foals. It was animal instinct that was to blame.
But then she felt her horsehair beginning to recede, along with her muzzle. Her hooves were gone. She felt naked.
And looking right behind her, she saw that the same had happened to Dan. He was still muscled and ripped, but horse no longer.
Which meant she was full of human sperm. Her brother's sperm.
The farmhand stepped out of the shadows. "Well, well. Looks like yere' back to normal. And just in time for the feast."