It’s true that horses (like people!) are innately crooked. And that crookedness contributes to your horse feeling uneven in the rein.
But not all of the unevenness in the reins comes from your horse. People often offer their horses uneven contact because their hands aren’t even. (One hand is closer to their bodies, one hand is higher, one hand is collapsed with the thumb pointing toward the withers etc.)
We need to be sure we’re part of the solution–not part of the problem.
So here’s an exercise I’ve found useful to learn the feeling (learning “feel” is always a challenge!) of offering an even contact.
Go in the walk, and put both reins in one hand. Make sure your arms stay elastic so that your elbows open and close as your horse moves his neck forward and back in the walk.
In this exercise, you have “one bearing on the rein”. You can ONLY have one bearing because both reins are in one hand. Imagine what that feels like in your horse’s mouth.
Now go back to riding your horse with one rein in each hand. But try to give him the same feeling he had in his mouth when the reins were in one hand.