Ok so if you do certain things to a horse it gets faster than its parents and doing the same thing with the affected horses makes THEIR offspring faster

How To Recreate-

You need-

2 Golden apples to breed

2 Horses

2 horse pens

1 Splash Potion of Speed

Reproduction-

Put 2 horses in the pen
Throw the potions at the horses
Quickly breed then put the offspring in the 2nd pen