Help with boss creation?

I have this idea that the 'boss' is destroying towns, but the players encounter it and it turns out that it's being controlled against its will by the true BBEG. I want something fairly strong to be the token, as it's supposed to be the BBEG, but what should I do for it? I'm thinking Aboleth BBEG, but is there something better to control a powerful token and destroy stuff?

Well, the method of control can be a plot device; there's no requirement that the real BBEG is even as powerful as the tool (of course, if you present your PCs with a mind controlled tarrasque and the PCs don't figure it out, your players may think you're being unfair).

Any moderately intelligent enemy could have a macguffin which lets them mind control a monster.

~Depending on level, the token could be the Tarrasque, a T-Rex, a devil or demon (EG a Balor), basically anything could work. For it to be a bit of a clue, you might be better off picking something which is traditionally more docile or good-aligned, which I am struggling to immediately suggest a specific creature for. You could have it be a pack of something which doesn't usually group together, EG the towns are being undermined by a pack of Bulettes which is really strange because they don't usually hunt in packs. The adventure could then go into the tunnels to find that a duergar is controlling them from the underdark to destroy the land above and sink their resource into the underdark for his minions to pillage without ever setting foot above-ground.

Quote from ThorukDuckSlayer >>

Any moderately intelligent enemy could have a macguffin which lets them mind control a monster.

~Depending on level, the token could be the Tarrasque, a T-Rex, a devil or demon (EG a Balor), basically anything could work. For it to be a bit of a clue, you might be better off picking something which is traditionally more docile or good-aligned, which I am struggling to immediately suggest a specific creature for. You could have it be a pack of something which doesn't usually group together, EG the towns are being undermined by a pack of Bulettes which is really strange because they don't usually hunt in packs. The adventure could then go into the tunnels to find that a duergar is controlling them from the underdark to destroy the land above and sink their resource into the underdark for his minions to pillage without ever setting foot above-ground.

 If you want something that is usually good aligned, you could have the BBEG be controlling something like a Solar with some sort of holy relic that glows with divine light while it is used. It would give the players a hint to it's importance that was as subtle as a flying brick, and give them a nice turn of events if they managed to wrestle the relic away and use it to break the BBEG's control effect. Plus....Solar's are scary foes to have to face, which makes the sadist DM in me smile.

Usually the arcane inclined are the ones to use this sort of technique for their evil plots. So some kind of wizard or warlock should control the creature, which could literally be anything, as long as it has the ability to cause destruction. And don't expect the players to pick up on the control thing unless you drop in easily observed clues and some local resources to help them decode it (eg. a library), with things like runes on the skin or glowing chains around their neck.