As Breanni of WarcraftPets points out, Withers is a relatively large noncombat pet when you first summon him and then shrinks over time, growing back to his full size when he's near a body of water. (We had trouble getting him to do his emotes, although we did see him noticeably shrink several times.) Unlike the Teldrassil Sproutling, which he resembles, he's got a worn, weatherbeaten look and the amusing size change process. Since he seems to be only available from an Alliance quest, he's most likely Alliance only. No word on Horde-only pets so far, but we'll keep an eye out.