After reading the book again, I have changed my first impression that the scaly baby was the part of V soul that was in Harry. I now think that it's what's left of V - the whole 2/8th theory so say you. Each of the Horcruxes, when destroyed, screamed and faded out - perhaps on some "train", but certainly on a different track than Harry would have taken. This "baby" was still there, not fading off into a scream. So I think it's Voldy in limbo with Harry, as would make since as V was on the ground with DM surrounding him when Harry came to, and Harry later warned V to repent as he saw what he was to become.