I was thinking that the Karzahni plant could 'evolve' into the Morbuzakh through some kind of mutation, or maybe devouring a Rahi/Matoran and integrating the physiology into its own, becoming more. That, or they're the same being from the very start to prevent confusion.
As to where to put it, I think Karzahni is a better pick (aesthetically) than Makuta, just based on the nature of the plant-things and how thematically similar it is to Karzahni. If we wanted the best of both worlds, the plant-thing could be a Kal Precursor, possibly a sibling or offspring of Tren Krom.
EDIT: I'll put it under Precursor for now, but this discussion is open to changes.