This work explores the use of active tails for steady-state legged-locomotion. Simple models are proposed which capture the dynamics of an idealized running system with an active tail. Analysis suggests that the control objectives of injecting energy into the system and stabilizing body-pitch can be effectively decoupled via proper tail design: a long, light tail. Thus the overall control problem can be simplified, using the tail exclusively to stabilize body-pitch. We show in simulation that models with long-light tails are better able to reject perturbations to body-pitching than short-heavy tails with the same moment of inertia. We also present the results of an active tail mounted on the quadruped robot Cheetah-Cub. The results show greatly improved forward velocity and reduced body-pitching and validate the long-light tail design: shorter, heavier tails are much more sensitive to control parameter changes.