This is a Mawile.

Mawile previously saw little use, being generally outclassed at most things. It could occasionally find a place on a Baton Pass team, but that was it.

But then it gained the ability to go go full-on power rangers on that shit, a badass ability, and proceeded to wreck the metagame and tear former top threats a dozen new assholes.

Any other pokemon who fell into such a slump that could be saved with a type change or a different ability?