Elder Olthoi Brood Queen.
Imagine a cross between the Alien Queen ("Get away from her, you *****!") and your typical D&D Giant Ant, only rather less fragile and with pretty good regeneration.
She's kind of like Mowen Udaun, except that Mowen does blugeoning damage and is weak to fire and pierce while she does piercing, bludgeoning and acid damage and is weak to bludgeoning and pierce. I doubt either of them could take the other down because they both regenerate too fast, but it'd be one heck of a fight.
An army of her? Sheeeeeeeeeee-it. One-on-one she'd stand a pretty good chance of corroding a T1000 out of action and making a full recovery. Sure, the air force could one-shot her with a big LGB, but those things are expensive. An army of that T-rex-sized, armour-plated, razor-clawed, acid-spraying psychopath? Even if none of them starts laying eggs, that's scary.
Of course, anyone who's been playing zangband or anything like it could have a really scary prospect lined up: the Green Worm Mass, Black Ooze, Vorpal Bunny or any other creature with the "reproduces" flag set. Just one Green Worm Mass somewhere on Manhattan would be grounds for destroying all the bridges and tunnels. Vorpal Bunnies would probably get off the island before you had time.