I've never thought about this before, but I have a bit of a strange idea for an explanation - the black thing that Ansem fights with is the hearless (normal, generic, everyday heartless) and Ansem is just the guise it puts up.
Think about it - the other people shaped heartless (Riku black, corrupt Clayton, dragon Malifecent, etc) don't have a black shadow attached, but the normal every day heartless DO have weird and wonderful attachments (Shield hound from hollow bastion, pot spider from agrabah, any number of bosses). So this is a bit more of a unique one, but thats not unusual since almost every world has at least 1 unique heartless, and many of them can transform when they take sufficient damage. The only confounding factor is that he can talk, and we don't see any other heartless do that, but that could just be another uniqueness - we do see others make noises, so his could just be more refined. Thoughts?
Hmm, I forgot about Birth By Sleep. Does Terra have a confirmes nobody? His heart was certainly strong enough to leave one behind. What if the Terra's nobody is the forward facing Ansem, and it paired up with the heartless shadow part?