stirrup
鐙 centers on a stirrup, the metal foot support used when mounting or riding a horse.
鐙 combines 金 (metal) with 登, which likely contributes the sound and suggests stepping up or mounting, fitting the stirrup's function.
The left side 金 is metal, and the right side 登 looks like climbing or stepping up. Picture a metal stirrup that helps you step up onto a horse.
For トウ, imagine a toe slipping into the metal stirrup: toe -> トウ, and your foot is secure as you mount the horse.