Ah, the evil bearing rein. You'd probably never heard of it before reading Black Beauty, but we bet you're an expert on it now. The bearing rein controversy was very real in Victorian England, and in this book it becomes not only a hot button issue, but also symbolic of the self-centered ignorance of humans.
Every time the reins are mentioned, we're reminded of the price these horses have to pay in the name of fashion: The rein holds their necks in a stiff, unnatural arch, hurting their backs, hampering their breathing, and making it far more difficult to pull heavy loads. "To my mind fashion is one of the wickedest things in the world" (10.9), says Beauty's fellow horse, Sir Oliver, and after reading about these reins, we're inclined to agree.
The bearing rein, then, stands at the crossroads of cruel treatment and the servitude the horses are bound to. Incapable of speaking for themselves, they are at the whim of whoever owns them. With a kind human on their side, this experience can be loving and positive, but without humane treatment, some horses are forced to endure painful physical hardship. It's not a big leap to think of the bearing rein as the shackles of working Victorian horses.