here is a simple instructable on how to make a chariot for your rat to ride behind your dog on, and all you need is a cardboard box!
find a box. not any box will do, but only the best box for your noble rat to ride into battle on!(i used a rice cracker box)
second cut off all of the flaps, and keep them for later use
for the wheels, take a cup and trace circles on the flaps. then cut out the circles and attach them using pins to the box. if the wheels arent staying up right, use cardboard to hold the wheels in place
cut out and attach cardboard strips to the side of the chariot at the appropriat length for your dog
add some support peices to hold onto your dog with
voila, your very own rat carrige!