This Alternate End takes place just after Twilight lowers ponykind's intelligence.
The ponies scattered throughout the continent, no longer bound by civilization or authority. They lived long, happy lives, and gave birth to horses that had never known Ponyville, Equestria, or magic.
And then one day, strange beings came to them. Let's focus on the ones that met Twilight's kin. Two beings walked up to Twilight's great-granddaughter, whose name would have probably been something along the lines of “Starsong” before, but was now basically “Scree!-Whinny-Snort.” The first of the two beings turned to the other and said, “So? Why Thak bring Thog here? Thog see nothing but horses.”
“Thog shut Thog stupid mouth,” Thak replied. “Thak have bright idea.”
“Is this 'Thak bright idea' as bright as fight crocodile?” Thog grunted, the closest thing he had to laughter until humor was invented.
“Hey, Thak plan would have work had Dorg not run away like mouse.”
“So, what Thak plan now, then?” Thog asked impatiently.
“Now? Thak jump on horse, ride horse everywhere.”
“Thak serious? Thog think of better idea in sleep time.”
“Thog know nothing. Thog think Thog so smart, but Thak not see Thog coming up with any ideas. Horse ride is good idea,” Thak groused.
“If Thak think so. But Thog not stand near to help when Thak fall off.”
“Thak not fall off. Thak have strong legs. Thak hold on.”
“Enough Thak talk. Thak jump on horse. Thak fall off. Thog watch, Thog not get hurt.”
“Here Thak go.” Thak braced himself, then jumped as high as he could. He grabbed Starsong's mane and used it to pull himself on top of her back. Starsong neighed her complaints, but otherwise, she didn't buck him off. “Thog see? Thak horse rider now!” Thak called down.
“Thog hold praise until Thak fall off,” Thog said.
“Go, horse, go!” Thak kicked Starsong's side, and she bolted off towards the sunset.
“Hey! Thak! Wait for Thog!” he said, running as fast as he could to catch up.
At last, when Thak had convinced Starsong to stop, he looked at Thog with an arrogant grin. “See? Told Thog ride horse was good idea.”
“Thak get lucky,” Thog grumbled. “So, now what Thak want to do?”
“Now, Thog and Thak get more horses, bring to cave.”
“What? Why?” Thog asked.
“Put horses near cave. Feed horses. Brush horses. Make collectible dolls that look like horses. Mate horses. Then, Thak have baby horse.”
“Then Thak have as many horses as Thak want! Thak smart!” Thog cried.
“Of course Thak is,” Thak replied. “Now, Thog and Thak find more horses. Much work to do.”
And that was how the first horses were domesticated. So remember, readers: whenever you ride a horse, you might be riding one of the great-great-great-great-a whole lot of greats-greatgrandchildren of your favorite MLP character!