This paper presents the design and development of a bio-inspired mobile robot called WR-3 (Waseda Rat No.3), as an experimental tool to study social interaction between rats and robots. According to the motion analysis of rats, their motion can be divided into two phases: moving and interaction. Therefore, a novel hybrid mechanism in which wheels are used for moving and legs are used for interaction has been designed to actuate the WR-3. Consequently, the robot can move at a high speed using its wheels and reproduce the rats' interaction using its legs and other parts. Based on the dimension and body structure of a mature rat, WR-3 has been designed with dimensions of 240×70×90[mm] and is the same shape as a rat. It consists of 18 DOFs in total: two 1-DOF wheels, four 3-DOF legs (including passive DOFs), a 2-DOF waist, and a 2-DOF neck. Preliminary interaction experiments with rats demonstrate that WR-3 is capable of reproducing interactions such as following, rearing, grooming, mounting, etc. similar to a real rat.