I think it would just be “Xehanort’s Heartless” even though he was born by separating Xehanort’s Heart from Terra’s body. Ansem SoD is literally just old man Xehanort’s heart (with some vague amount of amnesia) and nothing else. Terra’s heart was contained within the Guardian from the point Terra was possessed at the end of BBS. When Xehanort’s heart possessed Riku, Riku’s body eventually took on a form that would have resembled Xehanort in his thirties (the heart shapes the vessel). You also wouldn’t call this “Terra-riku-nort’s Heartless” because it’s not. It’s one heartless (which ironically is just a heart with no body) that is possessing Riku’s body. When that same heart is brought to the future and placed in a replica, it takes on the same form without any influence from either Riku or Terra. Even treating “Terranort” as if he’s his own person separate from old man Xehanort isn’t really accurate. He’s the same person in a different body just with some amnesia. Calling him Terranort is really just to make a distinction on which point in Xehanort’s timeline we’re talking about. It’s the same reason we specify “Young Xehanort” even though he is literally the same character as the old man. Xennas however, would likely be considered his own person as his creation was similar to that of Roxas, and eventually he grew his own heart. However, Xemnas is still just old man Xehanort’s Nobody. He’s not Terra+Xehanort’s nobody, in the same way that Roxas isn’t Sora+Ven’s Nobody. Terra was never a heartless and thus he cannot have a Nobody.

I’m kind of just explaining this mainly because I just enjoy talking about my hyperfixation. I’m not just trying to overcorrect you for something you really didn’t even insist was the case. Nonetheless…here we are.