rear back
To jerk or pull backward and stand or sit straight up. The horse suddenly reared back, throwing the rider to the ground. She reared back when I opened up the box containing my pet tarantula.
rear back
1. Lit. [for a horse] to pull back and up onto its hind legs in an effort to move backwards rapidly or throw a rider. (See also rear up.) The animal reared back in terror. The horse reared back and almost threw its rider.
2. Fig. [for a person] to pull back and stand up or sit up straighter. He reared back in his chair and looked perturbed. Tom reared back in his chair, waiting for something else to happen.