Morphological adaptation is vital for biological organisms navigating changing environments. While robots have sought to emulate this adaptability with adjustable body structures, practical robotic applications remain constrained by the complexity of integrating advanced materials, sophisticated control systems, and novel design approaches. This paper introduces a bioinspired quadruped robot featuring both a laterally undulating spine and posture-changing mechanism, specifically designed for adaptation in complex terradynamic environments. The robot utilizes a symmetrical parallelogram mechanism to precisely control its height and width, enabling it to navigate diverse terrains adeptly, avoid collisions, pass through narrow channels, and negotiate obstacles. Furthermore, the robot achieves stability through lateral undulation, which actively counteracts instability arising from posture changes. This ensures the center of gravity remains within its support triangle for the majority of the locomotion cycle, thereby obviating the reliance on intricate posture-stabilizing sensors or learning algorithms. The experimental results demonstrate the robot's capability to traverse both flat and significantly inclined surfaces (10° uphill and downhill), as well as successfully navigate confined tunnels, down to a narrow width. We observed notable variations in locomotion speed based on posture: certain configurations exhibited speeds that were up to 30% faster than others on surfaces with the least roughness, with similar trends holding for intermediate and maximum roughness. Furthermore, the robot demonstrates energy efficiency; while zero-degree posture showed a modest increase in average power consumption (around 18%) compared to others, the overall energy expenditure across various gaits remained consistently low. This work contributes to the development of versatile and autonomous robotic systems capable of operating in unstructured and unpredictable real-world scenarios, bridging the gap between theoretical adaptability and practical deployment in fields ranging from exploration to disaster response.