Animal movement is typically framed as an adaptive trait enabling exploration, foraging, escape, and reproduction. Such explanations describe the utility of locomotion but do not address why movement became evolutionarily necessary for animals while remaining absent in other major life architectures. This preprint proposes a foundational alternative: animal locomotion arose as a regulatory necessity rather than a behavioural innovation. Building on metabolism-first and regulatory-closure perspectives, the paper argues that once life internalised metabolic buffering—storing energy, concentrating reactions, and retaining waste—stationary regulation became unstable in heterogeneous environments. Movement emerged as a means of preserving internal coherence through spatial relocation. The framework unifies microbial motion, sperm motility, muscle contraction, and animal gait within a single oscillatory regulatory lineage. Muscles are reframed not as locomotor engines but as force-cycling regulators, while locomotion is interpreted as mobile homeostasis—regulation expressed in space. The paper presents testable predictions linking locomotor rhythm, metabolic state, and regulatory stability, with implications for physiology, origin-of-life research, and astrobiology.