Steven is Bruce's pet rabbit.
It is revealed in the DVD version of "We Love You, Conrad" that he keeps Steven at his bar. Bruce likes rabbits because someone can pull the ears back and make them look like Asians. When he demonstrates for Brian Griffin, Steven demands carrots and lettuce from Brian in an Asian accent.
Steven joins several other animals of the show as well as the cast in singing "Blue Shadows On The Trail" in "Wild Wild West".