Okay, now that Tera is staying I may as well comment the problem mons:
Any remotely viable team has at least two checks to this to nobody's surprise, Tera Fire allows Koraidon to get a nuclear STAB alongside sun, and while
exists, it's not too difficult to overwhelm with SD + Low Kick, especially as Desolate Land also triggers Orichalcum Pulse, so even if it's forced to switch out to set its weather later and blow holes more efficiently, nuking P-Don in itself is opening a wide hole for other mons to abuse.
is also a potent answer, but Koraidon can retain momentum against it with U-Turn or just nuke it on the spot with Z-Outrage. So far you're obligated to guess what set Koraidon is running as otherwise it can nuke a check and ease sweeping.
Personally I've been running
(Fairy) lately, and it has worked quite nicely to invalidate offensively pivoting Koraidons, while also doubling as a check to stuff like
and stuff like
and
in a pinch with Tera Ground, although it struggles with SD Tera Fire Flare Blitz, especially with the mind game of having to either KO it with Fairy STAB or Ground coverage, as picking the wrong move can cost the match.
Uber's equivalent of
right here. Geomancy in itself pushes this to the border already if it wasn't for stuff like
,
and
being common answers, except it can nowadays just brute force past them with the right Tera type. Electric ensures a OHKO on Ho-Oh with Thunderbolt after a Geomancy while also giving a far better match-up against Steels, meanwhile Tera Ground gives a better match-up against Primal Groudon, but at least it's forced to run Hidden Power Ground on such sets over Tera Blast as the deer is dexit'd.
still works as a hard-check, especially as a paralyzed Xerneas has about a 50% chance to fail Geomancy out of requiring two turns not being para hax'd, but it's hard to fit Blissey in general as it's naturally very passive.
Scarf sets aren't unheard of either, but TBH those are the most tame sets, especially as local caveman dragon
also runs Scarf fairly often and can easily chip it over time.
Oh yes, anyone that has tried ladder for a while knows how degenerate this snake can be. Zygarde is one of the few Pokémon in general that can afford setup and crippling checks on the same set out of only requiring a single attacking move to sweep (Thousand Arrows). Tera Fairy allows Ziggy to remove any of its relevant weaknesses to find even sturdy checks like
as setup fodder, it can even afford running Rest and just patiently wait to keep sweeping afterwards against teams without a potent special wallbreaker, which are inconsistent sometimes out of Glare, and I don't need to explain that the only mons that don't mind Glare (Electrics) dislike Thousand Arrows.
Arceus does the usual stuff, being the equivalent of
over here glueing all sorts of teams, and filling countless offensive and defensive niches with its high stats and movepool and item removal immunity. But Arceus-Normal sticks out a bit as Extreme Killer benefits a lot from Teras, Tera Normal means that Arceus gets a surprising offensive boost and can clean teams without a faster Ghost-type, which are rather uncommon especially since
already got banned by being dumb with Teras.
has to deal with a mind-game as picking the wrong move can cost you the mon and probably the match, while
can't KO Arceus from full and has terrible physical bulk (even less than
).
A hard-counter to the standard set exists with
(namely, it's immune to Extreme Speed and Earthquake), but Arceus has been running Ghost coverage lately specifically for it, let alone Tera Ghost too, and so overall even with the best options Arceus has quite limited counterplay and can easily pick it from the teambuilder. Note that I don't advocate to banning all Arceus formes, as much
has each forme tiered individually, and I feel that Arceus being forced to run Tera Normal to gain a STAB in Extreme Speed would make it more impractical, especially as then it'd be unable to stack it with its innate STAB for an Adaptability-like boost.
Primal Kyogre is a popular thing to mention when it comes to Teras, Tera Water Water Spout being able to just 2HKO even
speaks by itself, although it has the most obvious counterplay in
and to some degree the gimmicky Utility Umbrella
and extremely fat Water resists like
and
, plus common PP stalling strats especially with Pressure being common. Calm Mind sets also deserve a mention as they allow a healthy Primal Kyogre to overwhelm defensive answers, while also limiting offensive ones out of its already good special bulk. More offensive strategies of just wearing it down, especially for offensive teams that don't give it too much breathing room are also somewhat reasonable, and so I have mixed thoughts on
. The counterplay is fairly common and given its lack of reliable recovery and physical bulk to some degree it requires a good amount of momentum and prediction to blow holes and the like.
Other mons that considerably benefit from Teras but I wouldn't consider them as broken currently:
Kyurem Black has been the weird joke in OU since its introduction until Gen 8 as it got tools that pushed it too far from OU, namely Dragon Dance and Icicle Spear. Nowadays it can elect to run a Loading Dice to have a consistent physical Ice STAB, and Tera Electric provides it with a far better defensive typing while also enhancing its Fusion Bolt, in total giving it STAB BoltBeam, thus becoming a potent sweeper. A weakness to Stealth Rock combined with limited defensive utility and stuff like
being neutral to BoltBeam and forcing it out if it dares to Tera keeps it in check.
Besides being able to viably leave Teras at a side and just Ultra Burst into a potent sweeper, NDM benefits from Tera Water to handle stuff like
and
in a pinch, while more offensive sets benefit from a Tera to one of its STABs, or Ground to enhance its Earthquake and change its weaknesses, especially once it has triggered its Weakness Policy, but ultimately NDM lacks a bit on Speed and Special Defense, and isn't too difficult to force out or play around, especially with local
, even if still a quite potent mon in general.
Giratina-O fills similar niches as the regular
while also packing more offensive power from unresisted STABs alongside a Ground immunity in Levitate in exchange for bulk. Tera Steel is surprisingly underrated on this, it invalidates
sets without Fire STAB,
(Ground, Fairy),
,
,
without Superpower, and
, among others, while still checking
(Normal) decently, especially for Extreme Killer variants with Ghost coverage. However, this can't be used for free at all, scouting is often required for consistency, especially if Giratina is your only check to stuff like some
sets that skip Dragon STAB. Giratina-O lacking reliable recovery also means that you can't use it too recklessly either.
These are some of the most viable Arceus formes with a plate, and they all use Teras similarly, namely by swapping into another type to compress defensive roles depending on the match-up, sometimes also to just ease setup for Calm Mind or Dragon Dance sets. But what makes their usage of Teras not broken, besides the opportunity cost of using one of these over Extreme Killer, is that their usage is mostly defensive, which is far more manageable, and unlike
, they can't just cripple checks as they require dedicating at least one more moveslot for coverage to not become ironically passive.
Lunala has a quite terrible defensive typing and a weird speed tier that's easy to abuse even with Shadow Shield, and while Teras easily compensate, it's difficult at times to justify Lunala as nearly always wanting to Tera to be a reliable mon means a considerable opportunity cost, let alone flexibility on teambuilding, as much nowadays Teras are also used to compress counterplay in teams depending on the context.
Mons that barely use Teras:
Surprisingly Primal Groudon isn't that much of a Tera abuser. Tera Fire Eruption/Overheat isn't unheard of (as far ladder being ladder is concerned), but that kind of set is quite match-up fishy as Fire resists are everywhere in the meta thanks to Game Freak making Dragon the most common legendary type. Tera Flying is common as it flips the match-up against stuff like some
(Ground) and
sets, but becoming weak to Rock (notably including Stealth Rock) and Fire (losing the Fire resist is relevant as Desolate Land can backfire, heh) and overall having less defensive utility beyond that makes me consider such Tera type a liability than a benefit more often than not.
Ho-Oh in general is more of a utility Pokémon than a sweeper, and so it doesn't even bother with offensive Tera types. Given its duties include checking Ground-types without Rock coverage like
,
(Ground) and even
sets without Glare (Thousand Arrows hits neutrally the first time, so you can afford crippling or phazing it out), plus
without Dragon STAB and of course
, Tera-ing Ho-Oh is generally a liability as then it loses to a good chunk of mons it's meant to check, however, I've found some success on Tera Dark late-game (namely when I know I can afford that) to annoy
by luring its Z-Move after it uses Dragon Dance on the forced switch and phazing it on the spot, in turn becoming a solid check to stuff like
,
,
and the rare
.
Overall it's undeniable that Teras have considerably given shape to the meta at this point, and given they're still the status quo even after the suspect, I'd be expecting further tiering action on the current mons that stick out soon.
walls everything but
even without a curse and
/ 
healing 
