Gastrodon

Gastrodon holds a miniscule niche in DOU for its ability in Storm Drain, which allows it to redirect strong Water-type moves like Urshifu-R's Surging Strikes and Volcanion's Steam Eruption away from vulnerable teammates. Additionally, its typing and special bulk allow it to switch into strong Fire-type moves, such as Heatran's Eruption, and counterattack with moves such as Earth Power and Ice Beam. However, Gastrodon's typing makes it extremely vulnerable to Rillaboom's Grassy Glide. Additionally, Gastrodon is also fairly passive and has difficulty making a long-term impact on the game. Its effectiveness is also undermined by a lack of physical bulk, which also somewhat neuters its ability to counter physical Water-types that have neutral coverage. Gastrodon works best when paired with Pokemon that can take advantage of its ability to redirect Water-type moves. Diancie can gain some safety to set up Trick Room, while Fire types like Incineroar and Heatran also appreciate Gastrodon's ability to shut down threatening Water-type attackers. However, Gastrodon's 4x weakness to Grassy Glide means that it requires teammates that can switch into Rillaboom like Incineroar and Genesect. All in all, Gastrodon is overall a suboptimal pick that struggles to meaningfully benefit a team more than other options. Its weakness to some of the most potent Pokemon in the metagame means that facing a hard counter is extremely likely, and it doesn't provide enough utility for it to be worth it.