I think the in-game Pokemon that is far more trouble than it's worth, but still useful competitively is Hydriegon.
You can get Zweilous no earlier than Victory Road in any game. You'll catch it at around Level 50 and, haha, have fun grinding to level sixtygoddamnmotherfuckinfour to evolve this thing with Body Slam and Dragon Rush, missing so often with the poor accuracy compounded by Hustle that you want to throw your damn DS into the wall.
When you finally hit 327,680 experience points, you get to see Zweilous sprout another emo head and you think to yourself, "Shit, I could have easily taken down the Elite Four by now."
You'd think it's completely barren move pool would improve a little more substantially, but really, Dragon Pulse, Fire Blast and Dark Pulse are about all you can use on this thing.
I guess that doesn't matter, as you are dizzyingly over-leveled at this point.