Quadruped animals switch gait patterns with speed for energy-effective movement. This is similar to the phenomenon that excited natural vibration modes switch with vibration frequency in a multi-degree-of-freedom system. Therefore, in this paper, it is assumed that quadruped animals move by using the natural vibration of their own musculoskeletal systems. In the simplest rigid-body-link model consisting of one body and four legs, there are natural vibration modes similar to the gait patterns (trot, pace, and gallop) of quadruped animals. However, all the natural frequencies in the model exist near the natural frequency of the free leg and are accordingly different from the walking frequencies of actual quadruped animals. When a scapula and a pelvis are added to the rigid-body-link model on the basis of observations of quadruped motion, the natural frequency of the gallop mode used at high speed increases greatly and approaches the walking frequency. If the body characteristics of a horse are applied to the rigid-body-link model with leg joints, the natural vibration modes of the model are close to the gait patterns of the horse.