A horse that consistently leans on one rein more than the other is showing a stiffness or asymmetry in its body that prevents it from carrying itself equally on both sides — and the leaning on the rein is the symptom of that underlying asymmetry rather than a habit that can be fixed by changing what the rider does with just that rein. The fix therefore addresses the horse's whole-body straightness and suppleness rather than targeting the leaning rein in isolation.
The first step is identifying which way the asymmetry runs. Most horses are naturally bent slightly to one side — they hold their neck and body in a subtle arc that makes one rein feel lighter and one feel heavier. The horse leans on the rein on its stiffer side because it cannot yield through its neck and topline on that side as easily as on its more supple side. Lateral flexion exercises focused on the stiffer side — asking repeatedly for soft bend to the stiff side and releasing immediately when any softening is offered — are the most direct way to develop the suppleness that reduces the leaning.
From the saddle, the correction for leaning on one rein is counter-intuitive: soften and give with the rein the horse is leaning on rather than pulling harder against the lean. The horse that is leaning on the left rein has found a balance point against the rider's left rein resistance — removing that resistance by softening the left rein and asking the horse to carry itself without the support teaches the horse that there is nothing to lean against and it must organize its own balance. Simultaneously increasing the left leg aid encourages the horse to step under with its left hind, which addresses the root cause of left-side stiffness.
If the leaning is persistent and particularly pronounced to one side despite consistent suppling work, a veterinary assessment is appropriate — significant one-sided stiffness can reflect a physical issue including neck, back, or hind limb pain that suppling exercises cannot address.