I believe we need to suspect Thundurus-Incarnate for obvious reasons.
1) It has the fourth most powerful move in existence: Thunder Wave (first being Dark Void, second being Spore, and third being Power-Up Punch). It cripples almost everything in existence, with the exception of Mega Absol (never seen), Mega Diancie (really common), Manectric (left the scene a while ago after being very rare), Landorus-Therian (probably the most common Pokemon in the meta), and Mega Sceptile (rare). at 20% of its actual Speed due to Thunder Wave, the crippled Pokemon has a 50% chance to get off the needed attack. Good players fucking hate to get Thunder Wave spammed for obvious reasons.
2) It has Taunt to literally stop Trick Room and Tailwind immediately unless they have Fake Out. Sure, Fake Out is common utility, but after the Fake Out is used there are still other methods of stopping Trick Room, like your own Fake Out. Kangaskhan and Thundurus are a dynamic duo purely because of the opponent's inability to move letting Kangaskhan get set up. You can ladder to the top with both Kangaskhan and Thundurus as well as a Little Cup team in the back.
3) It is really bulky. Say we go back to our argument of having our own Fake Out and we say that we use Hitmontop to provide our bulky Fake Out and utility overall. This allows Thundurus to take more physical hits while it spreads the Thunder Wave love.
252+ Atk Landorus-T Stone Edge vs. 212 HP / 128 Def Thundurus: 294-348 (83.5 - 98.8%) -- guaranteed 2HKO after Leftovers recovery
^the calc without Intimidate (notice that Thundurus is still hanging on by a thread)
-1 252+ Atk Landorus-T Stone Edge vs. 212 HP / 0 Def Thundurus: 232-274 (65.9 - 77.8%) -- guaranteed 2HKO after Leftovers recovery
^the calc with Intimidate added as well as less Defense, which means Thundurus can tank a lot of Specially Defensive hits when Landorus-Therian is not around.
The bulk is sadly real for anybody who wants to challenge it. Specially oriented Thundurus is extremely versatile against Rain and even absorbs Thunderbolts from itself.
252+ SpA Ludicolo Ice Beam vs. 212 HP / 184 SpD Thundurus: 164-194 (46.5 - 55.1%) -- 14.1% chance to 2HKO after Leftovers recovery
My point is that Thundurus is going to take a lot before it goes down, and before that happens it's going to leave a lasting memory on the team for the rest of the game in the form of shitting on it with Thunder Wave spam.
4) It's very strong. I mean just look at the base Special Attack, 125 is almost as much as, well, Mega Manectric. It hits really hard and works for and with Rain excellently while also providing a good method of haxing opponents for all teams using Thundurus.
52+ SpA Thundurus Thunderbolt vs. 252 HP / 4 SpD Politoed: 270-318 (70.3 - 82.8%) -- guaranteed 2HKO after Leftovers recovery
Doing that to something very relevant in the meta game is something really scary for every rain team.
I think we should suspect it just because it's far too good at forcing coinflips, staying in, stopping Trick Room and Tailwind, and it excels at hitting hard.