I'm gonna post 3 that I think may fit the bill.
Lanturn, Seismitoad, and Druddigon.
Lanturn
Lanturn eats Mega Zard Y's hits for breakfast, and it eats Thundurus's for lunch.
(this is Smogon's standard VGC set)
244+ SpA Mega Charizard Y Solar Beam vs. 252 HP / 252 SpD Lanturn: 164-194 (70.6 - 83.6%) -- guaranteed 2HKO after Leftovers recovery
Pros:
+) Amazing and often overlooked defensive typing
+) Wide movepool
+) Great HP
+) Immunity to Electric thanks to Volt Absorb
+) No one ever uses it so no one knows what it does
Cons:
-) Poor defense and average SpD
-) Dies quickly to the likes of Mega Venusaur
-) In order to take Mega Zard Y's Solar Beam, it's gotta forgo non-attacking moves or special attacking power
-) No one ever uses it probably because it's kinda bad
Here's the set I would run:
Broturn @ Leftovers
Ability: Volt Absorb
Modest Nature
EVs: 252 HP / 4 SpA / 252 SpD
-Thunder Wave
-Heal Bell
-Scald
-Thunderbolt
Seismitoad
Seismitoad is an often overlooked Pokemon because Swampert's just better. However,
Swampert doesn't have Water Absorb. Seismitoad can hard wall a lot of Rotom sets and shrugs off most Thundurus sets.
Pros:
+) Wide movepool
+) Potential to run mixed
+) Not total Trick Room fodder
+) IT'S A FROG
Cons:
-) Defenses aren't that outstanding
-) Easy to force out
-) Poke it with a leaf and it will die
Here's the set I would run:
Seismibro @ Rindo Berry
Ability: Water Absorb
Sassy Nature
EVs: 252 HP / 80 Atk / 176 SpD
-Scald
-Earthquake / Toxic
-Rock Slide
-Ice Punch / Stealth Rock
And last but most definitely not least:
Druddigon
Druddigon positively screams potential. The opponent won't know what set you're running! Is it a Sheer Force Life Orb set? Is it a Mold Breaker Earthquake set? Is it a Rough Skin Rocky Helmet set? Your opponent most likely will freak trying to find out what the heck this thing plans to do. Mega Kangaskhan shudders at the thought of taking Rough Skin and Rocky Helmet both twice, and you can destroy it with Revenge.
Pros:
+) Movepool bigger than Wailord, with gems like Sucker Punch, the elemental punches, Gunk Shot, and Glare
+) Respectable defenses
+) Unpredictable
+) Awesome Attack
+) Rough Skin and Rocky Helmet cause massive recoil to Mega Kang
Cons:
-) Burn it and it is essentially useless
-) Slow as heck
-) You can only run one set
Now, I'm gonna be honest. I have no idea what set you should run. But whatever you do run, this bro's a force to be reckoned with.