What if a 3 day old human knew how to walk? I don't think that would look any different, because they physically can't do it anyway.

The first couple years of human development completely change the structure of the body. Walking is only possible after a significant amount of that process has happened, and the body keeps developing even after you learn how to walk.

A three minute old horse is both structurally and mentally prepared to run. A three year old horse will be taller and heavier, but not structurally different enough to change what walking is to their brain.

What a horse can never do as well as a human, is to learn a completely new behavior. Our brains are unmatched for flexibility in learning. Infant humans don't need to be born with the knowledge or the structure for waking. Both can develop together over time because our brains are able to develop new behavior.

The mystery here is the difference between a horse thinking "legs go" and a human thinking "legs that are just ready to hold me up, do what I see other people do, and don't fall over". We only have a vague linguistic model to express our understanding of the underlying complexity.