Question: why does xemnas look like terra?
Answer: Xemnas still looks like Terra because it is not 100% of Xehanorts heart, but probably just a large portion, as Ansem, being a dark being, requires less of a heart to exist (as with Ventus and vanitas). And with Ansem, he is closely tied to the heartless guardian, so he looks like a mix of Terra and Xehanort also.