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.