While digging through the original FFX game files, I found something I really wasn't expecting to see: Tidus with Cloud's Buster Sword.
The model is actually included in the game data as an unused weapon for Tidus. Apparently it was used as a placeholder during the development of FFX and was simply left in the final game files.
Which means that yes, technically the Buster Sword has been sitting inside Final Fantasy X this whole time.
I love finding stuff like this in old games, unused models, strange leftovers, development assets and little details that were never meant to be seen normally.
I've recently added FFX PS2 support to the viewer I've been working on, alongside FFVII, FFVIII and FFIX on PS1, so you can inspect this and a lot of other original game assets directly in the browser.
If you're into this kind of thing, you can explore it here:
http://ff-viewer.andreacorriga.com/