This is Lucky, a rabbit who surprisingly looks very similar to Ralph. Well, don't be surprised. In the show's history, Lucky was the original Ralph on A Day at the Zoo, being very popular until his contract expired and the studio heads quickly replaced him with Ralph. Ever since, Lucky had never been the same, plotting against and seeking vengeance on Ralph for what happened to him.
Lucky on the whole is a very twisted and evil character. He can be called the polar opposite of Ralph. Where Ralph is kind and good-natured, Lucky is cruel, relentless, and somewhat a glory-hound. He merely wants to be back in the limelight and will get there by any means necessary.