Read tuó for the verb 'to carry on the back', what a pack animal does (驮运 to transport by pack animal). For a person carrying something on their back, use 背. The noun reading duò means the load itself: 驮子 (a pack-animal's burden).
Left horse radical, the simplified silhouette of a standing horse, reform of 馬. It indexes 驮 in the equine family with 驹 colt, 骑 to ride, 驾 to drive. The word means to carry on a pack animal's back, so the radical names the beast doing the hauling.
Right side 大 supplies the sound: an older dà / tà reading drifted to tuó through early sound change. It also adds a quiet flavour, since a pack horse carries big, heavy loads, and 大 'big' fits that image. The same phonetic appears in 沱 (a branching river) and 鸵 (ostrich).
