He knew that he had bred the Zebra, and the instinct to protect her and any future foals was growing in his mind.
Over the coming weeks both Harry and Vicky gave in more and more to their new instincts. They would spend hours grazing in fields or napping in the warm sunlight. The change was taking over their minds, but they were comfortable with it, having long since given up on their human sides.
In a few days they hit stage 6, forgetting about being human in favor of being a stallion and a Zebra mare.
The government was forced to round up those who were still transforming and take them to animal shelters, zoos, and farms to be cared for until they could figure out how to deal with them. Harry was taken by a rancher and Vicky was given to the local zoo.
Harry wound up at a ranch with other horses that used to be human, and Vicky gave birth to her first foal at the zoo. Over time they completely forgot about their humanity and lived out their lives as a horse and a Zebra.
In time they even forgot what it was like to stand on their hind legs, but they never forgot about each other. The love between them stayed strong, and even though they were no longer human they knew that they were meant to be together, no matter what shape they had.
And so the two lived their life happily, even if they didn't remember what they were before. As far as they were concerned it was all for the best, who needed humanity when you could be a animal and live a peaceful life.