Steven is a stuffed dog, found by the Janitor, to replace Rowdy after Carla lost him. ("My First Kill") After J.D. and Turk found out, J.D. kept Steven so they both could have a stuffed dog. ("My Scrubs")
The two dogs are identical, except Rowdy is 48 inches from snout to tail and Steven, according to the Janitor, is only 46... "on a good day." J.D. can tell Steven and Rowdy apart by feeling their undersides ("Don't ask me how").