The Monark Bughemoth, a monster design inspired by the behemoths of the game Dauntless by Phoenix Labs.
I see the Monark as blundering and clumsy in its caterpillar form, it's fatty bulk wobbling as it galumphs about, throwing itself bodily at slayers and thrashing about to right itself.
As it takes damage and senses that it's truly in danger, it cocoons itself, its armored chrysalis leaking plumes of toxic aether. Finally it bursts free in its imago form, which is everything it's caterpillar form is not - graceful, agile, fast, and precise.