A 3/3 that quickly turns into a 1/1 isn't exciting. As a Tainted Mongoose, I feel like you should either make it bigger, or make it gain something else (maybe on Threshold), like Venom.
Agreed. Nydaeli, are you looking for a drawback to give the creature because he starts off as an untargetable 3/3 for 2? I'd say something more general would be the way to go rather than nuking this dude totally once you get threshold. How about:
Threshold: Other creatures you control get -1/-1
Ok, that's a bit binding. How about:
Threshold: At the beginning of your upkeep put a -1/-1 counter on target creature you control.
Either way, he'll act well alone but won't help out your team at all but he'll stay a 3/3 for 2.