Well, yeah, another one. I wasn't really a fan of "Terra + MX = Xehanort" myself, but after I discussed a few things with another KH fan I had to agree with it.
I will call MX - Xehanort, Terra - Terra nad Young Xehanort - Terranort.
Origin
This is the part I don't really want to dwell on. The most possible explanation is that Xehanort possesed Terra. I don't really know how or why did he do that.
Why do I think that Terranort is a result of Terra's and Xehanort's fusion?
1. Eye color
Terra had blue eyes, Xehanort - orange and Terranort - brown.
Terranort's eye color is a result of mixing the eye colors of Terra and Xehanort. Orange + Blue = Brown.
2. Hair
Terra and Terranort have very similar hairstyle. Terranort's hair color is obviously the same as MX's.
3. Facial features
Terranort's face is exactly the same as Terra's - just look at the first image and compare their eye shapes, noses and chins.
4. Personality
Terranort, according to Ansem The Wise, possesed superhuman talents and was clearly fascinated with darkness. This is most probably his Xehanort part.
5. Keyblade
Well, Xehanort was a Keyblade Master and Terra was probably a Keyblade Wielder. That's why Terranort was able to open the door in Ansem's The Wise castle's basement.
6. The Room Of Sleep and The Room Of Awakening
Terranort made The Room Of Sleep. Remember when Xemnas was talking to Aqua's armor in there? That's probably the part of Terra's mind that was left in Terranort and that's why he called Aqua a friend. This is also the reason why Xemnas is looking for the Room Of Awakening - he wants to find Ven. Maybe Xemnas (or rather the part of Terra that's left in him) was looking for a way to bring them back to life (if they died, of course)?
Most of this stuff was already told, but I wanted to gather it in one place. Oh, and sorry for my grammar.
I will call MX - Xehanort, Terra - Terra nad Young Xehanort - Terranort.
Origin
This is the part I don't really want to dwell on. The most possible explanation is that Xehanort possesed Terra. I don't really know how or why did he do that.
Why do I think that Terranort is a result of Terra's and Xehanort's fusion?
1. Eye color
Terra had blue eyes, Xehanort - orange and Terranort - brown.
Terranort's eye color is a result of mixing the eye colors of Terra and Xehanort. Orange + Blue = Brown.
2. Hair
Terra and Terranort have very similar hairstyle. Terranort's hair color is obviously the same as MX's.
3. Facial features
Terranort's face is exactly the same as Terra's - just look at the first image and compare their eye shapes, noses and chins.
4. Personality
Terranort, according to Ansem The Wise, possesed superhuman talents and was clearly fascinated with darkness. This is most probably his Xehanort part.
5. Keyblade
Well, Xehanort was a Keyblade Master and Terra was probably a Keyblade Wielder. That's why Terranort was able to open the door in Ansem's The Wise castle's basement.
6. The Room Of Sleep and The Room Of Awakening
Terranort made The Room Of Sleep. Remember when Xemnas was talking to Aqua's armor in there? That's probably the part of Terra's mind that was left in Terranort and that's why he called Aqua a friend. This is also the reason why Xemnas is looking for the Room Of Awakening - he wants to find Ven. Maybe Xemnas (or rather the part of Terra that's left in him) was looking for a way to bring them back to life (if they died, of course)?
Most of this stuff was already told, but I wanted to gather it in one place. Oh, and sorry for my grammar.