One thing of important note is that the horse the man was riding didn't become an anthromorphic horse itself, not did it become sentient, and nor did it merge with the man riding it.
There was however, one change, the horse's muscle mass and bone strength increased, enabling it to continue to carry the now larger and heavier anthromorphic horse that now rod it.
For any who saw, it would have been strange seeing a biped horse riding a quadruped one, though the rider having the light of intelligence in his eyes.
It was nearly hypnotic seeing their tails swish together as they rode down the road. His hat more or less staying on him unlike the rest of his clothing, even if it was annoying with the new position of his ears. He actually took the hat off and looked at it, but two ear holes appeared in it like they had always been there and the still naked horse man calmly put it back on, wondering what the annoyance had been before.
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One thing the brothers realized was that it didn't matter to them if they were covering themselves or not, they felt no shame or embarrassment either way. And this actually disturbed them more. There was also this feeling of 'oddly standing out' among their fellow herdmates due to them covering themselves. Ick.
Their elder had shown no concern whether they were wearing the loincloths or not. But for the brothers, they chose to resist these feelings while they could, even if their perceptions had been protected from being altered, they chose to rebel against these new feelings and lack of other feelings, due to the cascade they themselves had brought upon the world.
The Butlers had chatted about going down one of two routes at the diner apparently, but hadn't decided on which one. Their only logical move was to stop at the next place now inhabited by naked horse people and ask for more clues. At least whoever they were asking acted in a perfectly cooperative and friendly manner.
The brothers didn't want to think about their fate if this change could not truly be contained. But if it couldn't, at least stallions weren't murdering each other over mares indifferent to who was the winner. That was a good thing at least.
If the three hadn't been in a rush to find the root of the transformations that was the married couple, and if they had been there to witness the changes. They'd have noticed two men of different ethnic groups glaring hatefully at each other, but after become a palomino and a chestnut, began politely playing a friendly game of cards.