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").
Behind the Scenes
Steven was actually acquired for the show in case something would happen to Rowdy, after an episode where Rowdy was hidden in a ceiling.