@Spritejunkee
That Marco turns into a monster.
Like, part of Eclipsia’s endgame would be turning mewmans and possibly beyond into irreversible monsters, and Marco becomes an unwilling test subject to this. As a callback to Monster Arm, he becomes a fully anthro’d octo-beast and this does include the return of monster arm, but his influence is curbed by the personalities of his other, more upstanding limbs(points for biology reference).
He spends several episodes dealing with this change before managing to retain a sense of normalcy by figuring out his innate octopus mimicry(which really just looks like shape shifting; another bio-joke) to look more like his original form.
And this arc also serves as a means of drawing a parallel to Eclipsia again, should one believe StarCo is our endgame.
I always thought that
Monster Love line wasn’t quite the the throwaway it seemed.
So, that’s my ramble, do what you will with that.