Jabba's original C&C section was short and sweet, I've reworded it a bit and added a few more checks.
If there was an award in OU for "most difficult Pokemon to switch into," here's your guy. Slowbro is probably the best check you'll find in OU, although a Stone Edge boosted by a Choice Band or a Swords Dance boost
will hit like a truck. Gliscor is also a good check, though
it'll be taking a ton from Stone Edge from those sets. Mew, Reuniclus and Jellicent also fall into the similar category
. There is light at the end of the tunnel though; Terrakion needs to hit two consecutive
Stone Edges to KO any of these Pokemon. Hippowdon can also tank a hit and Slack Off. You may be inclined to use Skarmory as your Terrakion counter, but despite its good Defense, Skarmory is ravaged by a Close Combat from the Choice Band or Swords Danceand the Swords Dancesets, allowing it to only deal with Terrakion that lack Attack boosts.
Terrakion can be hard to check because of its great Speed. Scizor and Azumarill are two Choice Band users that can stop any Terrakion set dead in its tracks with their STAB priority attacks. Infernape can also do a good chunk of damage with Mach Punch; Conkeldurr can do the same and also, when
at high health, can survive a non-boosted Close Combat and go for the OHKO with Drain Punch after Stealth Rock. Latios and Tornadus can outspeed and hit it hard with Psyshock or Hurricane respectively. Gengar and Starmie can smash it with Focus Blast and Hydro Pump, whilst Espeon can hit it hard with Psychic. Gengar deserves a special mention for being able to Disable Stone Edge, crippling Terrakion instantly. Dugtrio does what he does best and can trap Terrakion with Earthquake. Wobbuffet knocks Choiced versions of Terrakion out of the park and will make them think twice before spamming Close Combat. Choice Scarf users such as Rotom-W, Landorus and other Terrakion are also fantastic checks.
Ironically, Terrakion's best counters litter the lower tiers. Examples being Golurk, Nidoqueen, Claydol and Tangrowth. If you're feeling desperate, then one of these is your best bet.