We show that modeling directional kinematics is especially important for understanding spatial patterns of behavior in an arena. In rats, although patterns of locomotion appear to be under sensory control, they may be largely if not entirely explained, during early locomotor development, by the directional kinematics of random movement together with morphological/environmental constraints. Different relative probabilities of forward movement versus lateral movement (lateral: forward ratio; LFR) have dramatic consequences for the patterns of movement of modeled agents in an arena. These patterns of movement, produced by rigid-bodied agents, are also modified by increasing morphological realism in our model, wherein agents are able to bend and flex like rat pups. For both morphological designs, agents run with empirically derived LFRs matched metrics of movement for 7-day-old pups very well and 10-day-old rat pups fairly well. We conclude that LFR is an important parameter in modeling (a) patterns of dynamical behavior in an arena and (b) trajectories of behavioral development.