OC

Nara accidentally fell into the murky sea in Don's Realm where all the hungry, spooky nightmarish crocodile beasts roam and Don jumped into the water to save her and put her in a blanket.
My current headcanon is that Don changes form when he's submerged in that specific water. But the details of the circumstances for this form could change, not sure yet, but he definetly can't change form everywhere like he pleases.