Spectrier has been an extremely restrictive presence for teambuilding due to its offensive prowess, set up opportunity and incredibly high speed. The horse boasts an incredible base 145 Special Attack, the third highest out of any Pokémon allowed in OU following Xuriktree and Blacephalon. However, despite its stronger predecessors, Spectrier is the only one of these Pokémon considered highly viable in OU due to its typing and exceptional base 130 speed. This allows it to outspeed the vast majority of non-Scarfed and unboosted Pokémon, while faster Pokémon like Zeroara and Dragapult struggle to deal with it defensively. The horse also has access to its signature ability Grim Neigh, which grants it a Special Attack boost after KOing a Pokémon. This allows for snowball potential, enabling Spectrier to gain traction quickly. With that being said, Spectrier is not without flaws. Spectrier does have a major downside in its incredibly shallow attacking coverage. However, the combination of its exceptional stats and typing makes it incredibly difficult to play around and forces teams to run Pokémon that would not be as common if not for the horse.
Spectrier's primary set revolves around setting up for itself and sweeping. The main moves include Substitute and a boosting move in Calm Mind or Nasty Plot. The third move is Shadow Ball, a solid 80 base power STAB move, or Hex, a 65 base power STAB move that doubles if the target has any status. Whether or not Spectrier uses Hex is reliant upon its final move. For it's final moveslot, Spectrier typically chooses between Will-o-wisp and a coverage move that would allow it to hit Normal types. A second attacking move would usually be Dark Pulse, but people have used Hyper Beam as well to get important KO's on Hydreigon. The goal of this set is to set up on a Pokémon that is threatened out or cannot touch Spectrier. This can be achieved through Will-o-wisp onto Pokémon such as Mandibuzz and Ferrothorn that may struggle to break a Substitute after being burned. Generation 8's removal of the move Pursuit allows the horse to freely spam status or attacking moves without risk of being punished. Spectrier then boosts its stats, and a boosted Spectrier behind Substitute is incredibly dangerous in most situations. The Will-o-wisp set can be walled by Normal types, but what makes Spectrier truly annoying and difficult to prepare for is that your team might not be prepared for the specific set your opponent brings. For example, you can have Shadow Ball Blissey as your Spectrier counter, but that set would lose to Calm Mind Dark Pulse due to failing to break its sub. You could also use the simple Teleport Blissey into a Pokémon that threatens Spectrier out tactic, but with Pheromosa's departure, it is not easy to pivot between U-turn and Teleport around Spectrier. Similarily, Hyper Beam can effectively KO many common checks such as Hydreigon and Mandibuzz after minimal chip damage on them. Due to this, Spectrier is incredibly restrictive in the sense that it can pick and choose its counters. It should be mentioned that the other faster Pokémon in the tier also struggle vs Calm Mind Spectrier, with Dragapult having to trade its life for a Thunder Wave or damage and Zeroara losing most of its HP breaking Substitute with a Physical move. Before the rise of Substitute Spectrier, the most commonly spammed set was Choice Specs. No one can deny how strong Choice Specs Spectrier is in the metagame, oftentimes forcing people to use Hydreigon and Normal types. Furthermore, if paired with status spreading Pokémon, Spectrier having an instant Choice Spec boosted Hex was an insane threat to deal with, even being able to put a major dent in its most common counter, Hydreigon. Choice Specs was used for raw breaking, but its popularity has certainly decreased since people discovered how good boosting was.
The issues listed above encompass the main reason why Spectrier can be seen as too much for the OU tier. One may argue that it actually struggles with breaking vs teams with Hydreigon or Normal types, but it still has a move pool capable enough to break either. Despite this, the fact that every balanced or bulkier team is required to use a bulky Dark type or a Normal type speaks to volumes about Spectrier's presence in the tier. It is comparable to the past Dracovish, which either dented a hole in any team or was walled by Seismitoad. Additionally, a key similarity between the two is that the dapper fish's partners can set it up easily through luring Seismitoad or chipping the toad endlessly. What one really has to consider about Spectrier is the fact that it is a six versus six team game. Sets ranging from more extreme moves such as Focus Punch Nidoking to common threats such as Heatran's combination of Magma Storm and moves like Taunt and Toxic support Spectrier easily. There is infinite support for the horse through surprise breaking and status spam, all to set up the horses in the back.
However, Spectrier is clearly not a flawless Pokémon. While its typing provides solid offensive presence, only being resisted by Dark types and not being able to touch Normal types, but it also makes Spectrier weak to the all too common move Knock Off. Many Pokémon in OU run Knock Off, such as Clefable, Landorus-Therian and Toxapex. If Spectrier ever receives a flat Knock Off, its long-term presence and effectiveness decreases exponentially. Having Leftovers removed limits its boosting ability and also forces chip on it through hazards, status, and weather. Additionally, the prominent boosting sets start off weaker due to EV's being invested in bulk for more reliable set up. If this is called out early, Spectrier could struggle to gain its footing and prepare a sweep. Additionally, while it has incredible offensive stats, Spectrier is relatively frail with a base defense of 60. This can be voided by using Will-o-wisp and investing into HP and defensive stats, allowing it to maintain its Substitute. However, it also means that Spectrier can practically never switch into any moves. As mentioned numerous times, it also has common answers such as Blissey and Hydreigon, which can be used to counter other prominent threats in the metagame. Blissey is a complete Special sponge, easily switching into the strongest attacks from Pokémon such as Magearna. Hydreigon is another frequently used Pokémon, often used to counter a major metagame threat in Heatran. These Pokémon also have their own way of dealing with the horse with Shadow Ball and Snarl on Blissey and Hydreigon respectively. However, similar to how Spectrier is not flawless, these "counters" are not as well, and can be annoyed and dealt with through Spectrier's teammates. The combination of its restrictiveness and sweeping potential stated previously is the reason the OU council has decided Spectrier is worthy of a suspect test.