I LOVE that. I'm going to make this weird and a little too intense, is that okay? Ok, the existential dread of a monster transformation is so yummy. It's very easily applied as a metaphor for anxieties surrounding stuff like puberty and changes in personality/priorities as you get older and grappling with gender/sexuality identity, many things, but it can be played as straight-up stomach churning body horror too and I'm a rather big fan of that as well.

Let's run with Gunwook as the one transforming, like you said, bcs I think he suits the themes, specifically those surrounding fears of growing up and now that alters people's perception of you. Hm, does he know what's happening or no? Either way he'll be undergoing changes he doesn't understand. Even if he thinks he knows what to expect, there's no way to prepare yourself for what it really feels like to have your nails fall off and claws slowly erupt like wisdom teeth in their place. No one could ever tell you how it'll feel when you start losing control of your mind to new animalistic instincts, how it'll feel when you unknowingly speak your last words to the man you love and how you'll pray he won't hate you all while knowing you wouldn't be able to blame him if he did.

But Matthew would never leave him, no matter what, even if it means keeping him locked away so he can't get himself or anybody else hurt. He'll keep him comfortable, bring him food and water, sing to him from outside the room and tell himself that Gunwook snarling along to the melody means deep down he knows he's still loved. He'll always be his baby.