Definitely symbiont. Have to go break out MM2 again...
#1-3 seem easy enough.
#4 could be a bit tricky.![]()
So...
Diminutive plant?
None of the other symbionts are mindless, and it could mess up the Ego score mechanic most use to affect their host. The lowest Int score among them is 5.
Its a shame this isn't for Pathfinder, as it would make a wonderful affliction.
EDIT: This might work better as a simple plant monster, rather than a symbiont. It reminds me a bit of the ciruja plant in Dragon Compendium.