Robot Sheep

I am attempting to build robotic sheep that follow a radio becon mounted on a shepards crook. 

Currently I have 1 failed frame, thinking that my motor speed is too high (RPM) for the steel and Lead acid mounted on top of it. Frames for the 2 larger sheep are still being produced and the powertrain is currently coming togeather. The goal with those are "Follow Me" tech that can carry a significant weight (think Cooler).

I am using 2 250W 24V scooter motors controled with an arduino. I have not settled in on a motor controller yet but I was thinking a sabertooth, but I am unsure of the amprage needed. In reality these cannot go faster than 5mph, and would be going more like 2.5-3 (walking pace) anyway.