Gyarados

Gyarados clings to RBY OU relevance thanks to a combination of massively inflated stats, expansive coverage in moves like Thunderbolt and Blizzard, and a unique typing that makes it the only Water-type immune to Ground, letting it switch in on a predicted Earthquake with impunity. That typing lets it synergize with Pokemon like Chansey, Gengar, and Jolteon, which not only appreciate this immunity, but also help patch up its own Thunderbolt weakness. Gyarados's attacking stats are extremely high, allowing it to achieve feats such as 2HKOing Tauros and 3HKOing Snorlax with Hydro Pump, letting it win against them one-on-one. Plus, it's bulky to the point that even a Thunderbolt from Starmie fails to OHKO it without a critical hit. It's not exactly slow relative to RBY OU either, outrunning Pokemon like Cloyster, Dragonite, and Victreebel while threatening them with its coverage. Overall, Gyarados can be seen as a battering ram that blows past paralyzed teams.

However, while it may have been a big threat in the early days of the generation, Gyarados fell out of favor due to its typing giving it some horrendous weaknesses. While its Ground immunity looks enticing to switch in on Rhydon, it also takes huge damage from Rock Slide: its bulk isn't as good as it looks. It also gets a quadruple weakness to Thunderbolt and neutrality to Blizzard and Psychic, making it hard for Gyarados to get in against the special attackers of the tier and forcing teams to make up for it. If paralyzed, this gets even worse, as Gyarados's lack of reliable recovery makes it very easy to wear down as is. Even offensively, because of a criminal lack of physical STAB moves, Gyarados struggles to damage specially bulky Pokemon like Chansey and Gengar. Finally, Gyarados has issues distinguishing itself from Lapras outside of its typing and superior Attack, which has the same coverage and higher overall bulk. As a result of these flaws, Gyarados is a very rare sight in RBY OU, often requiring extensive support from a dedicated team to function properly, but it remains a serviceable pick in the hands of an experienced player.