Tentacruel

Tentacruel's great Special Defense, Speed, and Water typing give it a niche as a solid check to special Water- and Fire-types. Tentacruel critically has access to Rapid Spin, and it's a grounded Poison type, letting it absorb Toxic Spikes upon switch-in, which separates it from the other prominent Water-type spinner of the tier, Starmie. In addition, Tentacruel's unique mix of utility moves, such as its own Toxic Spikes and Knock Off, make it a very appealing choice for stall teams and further differentiates it from Starmie. Knock Off is especially valuable, as it reduces Tentacruel's passivity by crippling bulkier Pokemon and Choice item users; it also complements Toxic Spikes, as bulkier Pokemon tend to run Leftovers, which would otherwise mitigate the poison damage. Tentacruel's Poison typing gives it an important resistance to Fighting, letting it check Fighting-types with Grass-type coverage such as Breloom and Infernape, as they typically lack the options to hit it super effectively.

However, Tentacruel is still a rare sight in OU with fairly fringe viability. Despite having Knock Off, Tentacruel is fairly passive due to its mediocre Special Attack stat and struggles to damage the main spinblocker of the tier: Rotom-A. It's also weak to the omnipresent Earthquake, meaning it can't check powerful physical attackers such as Flygon, Swampert, Dragon Dance Tyranitar, Dragon Dance Gyarados, and Dragon Dance Dragonite, unlike most other defensive Water-types. In addition, many common defensive Pokemon Tentacruel would want to spin or set up Toxic Spikes on can potentially 2HKO it, such as Gliscor, Hippowdon, Quagsire, and Rotom-A. Above all, Tentacruel's low physical bulk, lack of recovery, and crucial vulnerability to Spikes and sand mean it struggles to safely get in and actually utilize its support movepool. Even resisted attacks such as Breloom's Focus Punch and Gyarados's boosted Waterfall can 2HKO it after entry hazard damage. Tentacruel has high base Speed, but it needs all the bulk investment it can get, so it is commonly outsped and then KOed by strong attackers. While the other viable Rapid Spin users, Starmie, Forretress, and Donphan, don't clear Toxic Spikes upon switching in, their longevity is significantly better; the former has reliable recovery and Natural Cure to stave off status, while Forretress and Donphan are immune to sand and much more physically bulky. Additionally, the ability to absorb and set up Toxic Spikes isn't hugely valuable, as stall teams commonly respond to that threat by running multiple Pokemon immune to them, such as Clefable, Jirachi, and Skarmory.