A collapsible trailer for exercising or transporting an animal. The trailer is attachable to a vehicle, such as a bicycle, and features a floor capable of retracting into a running position where an animal is able to run on the ground and deploying into a riding position where an animal can ride without having to exert itself. The trailer is further capable of collapsing utilizing hinged connections between trailer sides in order to have the trailer fold flat to occupy a reduced floor area.
