Thundurus can indeed be a very troublesome Pokemon to deal with once it gets its time to wreck havoc, however there are enough Pokemon in OU that can reliable check it while retaining their viability in this tier.
Hippowdon
Hippowdon is a great check for Thundurus-I on Sand teams, most notably the mixed wall and specially defensive variants, as they avoid the 2HKO from HP Ice and take like 0 damage from all of its other attacks. With great bulk and access to recovery and Stealth Rock, Hippowdon will commonly find itself acting as a very useful asset on sand teams.
Besides checking Thundurus, Hippowdon is a great check to the likes of Grass Knot-less Mega Metagross, Mega Lopunny, Mega Gallade, Tyranitar(Mega), Excadrill etc.
- Requires a spread of at least 252 HP / 112 SDef to avoid the 2HKO from LO Thundurus-I's HP Ice and Mega Manectric's HP Ice
Conkeldurr
AV Conkeldurr is a decent check to Thundurus-I, but cannot switch into it as often as Hippowdon can due that thing's great offensive prowess. I believe we all know that AV Conkeldurr is perhaps most known for its great ability to check Rotom-W and is generally a great status absorber as well as a good check to Bisharp and the pink blobs. Furthermore, it has the ability to strike Thundurus-I extremely hard with Ice Punch
Raikou
Like, do you even bop. Raikou is arguably one of the best offensive checks to Thundurus-I, while taking very little damage from its Thunderbolts, it strikes it down with its own. It also does a fantastic job at swallowing up them Thunder Waves and other then that, is a questionable check to Talonflame and overall great asset on fast offensive teams
Amoonguss / Mega Venusaur
Thundurus hates to go up against Amoonguss, mainly due to the simple fact that its attacks do very limited damage to it and it risks getting spored and crippled by Amoonguss in return. After it has been spored, Amoonguss can then commence with depleting Thundurus' health with its attacks, or have a teammate deal with the crippled threat. Other then that, Azumarill, Keldeo, Clefable etc... do not appreciate facing Amoonguss either, due to its ability to undo their boosts with Clear Smog
Celebi
Celebi acts in a very similar way to Amoonguss, just that it doesn't have access to spore, can set up on it or can simply fire off stronger attacks at once
Latias
However you must take note that not all Latias can deal with every type of Thundurus, as a mixed Thundurus with Knock Off will bop Latias into oblivion, however, bulkier variants of Latias are an honourable mention. Latias only requires 72 EVs in HP to avoid the 2HKO from Hidden power Ice and 104 to avoid the KO from a possible LO Knock off from 76 Atk Mixed Thundurus-I after Stealth Rock damage and can then proceed to roost of the damage and then annihilate Thundurus with any of its great attacks. Latias is also a very great check to a plethora of Pokemon in OU, such as Amoonguss, mega Venusaur, Keldeo. Rotom-W etc.
Landorus-T (Scarfed)
Despite it not being able to switch in on Thundurus' HP Ices, Scarfed Landorus-T can easily revenge kill it or with great prediction switch into its electric type attacks or Thunder Waves and proceed to kill it off with Stone Edge.
Landorus-T is overall a great Pokemon and can usually do well on pretty much any playstyle.
Weavile
All Weavile needs is for Stealth Rock to be up and it will be able to KO Thundurus-I with Ice Shard while not having to fear being Thunder Waved as well
Generally, having stealth rock up usually makes life easier when dealing with Thundurus as it will gradually wear it down
