#behold my weird theory that byleth could have transformed

[iturbide]

*wakes up in a cold sweat* one day during the war, Enbarr's palace collapses as a furious dragon breaks through, and though Hubert suspects it was Rhea escaping, the look on Edelgard's face as they flee says otherwise, the implication her being that at some point during the timeskip, the Twisted had found Byleth and were experimenting on them, only to tweak things a little too far to the left and turning Byleth into a fucking Dragon *passes the fuck out*

okay so I know that this is about like actual canon alternate timeline but I feel the need to mention that Byleth going dragon is a key plot point in the Pre-Timeskip Fix-It

please be warned there are spoilers hereafter

Okay so while we know that in canon most of our known dragons have lost the ability to transform in one way or the other (Seteth and Flayn can't go dragon, Macuil and Indech don't seem able to go human), Rhea still can and does transform at least once if not several times over the course of the game. While Rhea intended for Byleth to become an avatar of Sothis, they're actually the product of a human and a homunculus of sorts: Jeralt was fully human with his Major Crest bestowed by Seiros herself, and Sitri was made by Rhea and proved to be a failed avatar herself. We have no idea if Sitri would have been capable of transforming or what the detailed circumstances of her creation/birth were, but ultimately Byleth is the product of a union between a human and a (pseudo-)dragon.

Now, on top of all this, Sothis does end up merging her powers into Byleth, which changes their hair and eye color to the classic draconic green of the Fire Emblem dragons. It's entirely possible that the merge might have unlocked Byleth's capacity for transformation, along with things like their suddenly decelerated ageing -- but in canon, Byleth is very swiftly knocked out of commission for five years by either a blast from Rhea (CF) or Thales shooting them off a cliff (all other routes).

Now, my weird little plot bunny for Pre-Timeskip is that unlike any of the canon routes, Byleth manages to stay awake, escaping the monastery before the ritual in the Holy Tomb along with the students. This is important, because it means that the changes that started with the merge continue on, rather than getting suddenly aborted by a traumatic incident that necessitated hibernating to ensure survival: because all non-essential bodily systems got shut down, the factors that would have continued altering Byleth's biology also got aborted -- and unlike many others, they didn't restart after Byleth woke up, possibly because they were different enough from Byleth's original human biology and hadn't gotten far enough along to trigger a need to complete it.

This means that while Byleth and the students are going on their merry way and evading the Knights of Seiros, that change is still on-going. Byleth's body is trying to go through a change similar to a young dragon making their first shift from human guise to true dragon form -- and that gets interesting, because they're heading north into the Kingdom where it is very cold most of the year and unbeknownst to everyone else, Byleth is scaling out. Scales aren't exactly great at retaining body heat, meaning Byleth starts slowing down and struggling a lot more until they figure out what the problem is -- at which point Claude is put in charge of the professor because he knows wyverns right so he's clearly going to be the resident expert.

(Claude is not the resident expert. He is, in fact, fully aware that he is out of his depth. Which is why as soon as they reach Fhirdiad he contacts Flayn and sets in motion a comedy of errors.)