Here's a Pokemon that has also benefited from Zygarde being gone as well as one that (I believe) challenges Tangrowth as the Regenerator Grass-type option.
Amoonguss @ Black Sludge
Ability: Regenerator
EVs: 248 HP / 44 Def / 216 SpD
Calm Nature
- Spore
- Giga Drain
- Hidden Power Fire / Sludge Bomb
- Stun Spore / Toxic
Rotom-W had always usually been paired with Grass-types, and Amoonguss is no exception. Amoonguss easily soaks up Grass-type moves for Rotom-W and can also handle Electric-type moves that would otherwise take a chunk out of Rotom-W, along with other types of moves like Fighting. In return, Rotom-W can easily switch into the Ice-, Fire-, Flying-, and Ground-type moves that damage it quite a bit. What Amoonguss has over Tangrowth, however, is the ability to fit Spore and Stun Spore, let alone having a perfectly accurate sleep-inducing move, on a single moveset. Both of these moves help Rotom-W by being able to pivot with Volt Switch safely if it wishes to not slow pivot. Amoonguss is also able to take on Fairy-types fairly better with its Poison typing, most notably Fleur Cannon from Magearna. Will-O-Wisp from Rotom-W only bolsters Amoonguss's walling capabilities. However, this core will be fairly weak to Psychic-types like Mega Alakazam and Tapu Lele. Even if Tangrowth is considered the better Regenerator Grass-type, Amoonguss does have its advantages in the ability to use two status moves effectively and the addition of resistances to Fairy- and Fighting-type moves and less weaknesses overall.
tl;dr Amoonguss can eat Grass-, Electric-, Fighting-, and Fairy-type moves that would otherwise hurt Rotom-W while also having really nice utility in the ability to run two status moves in its set, which help Rotom-W to pivot more efficiently. As Tangrowth cannot do some of these, Amoonguss can be a viable option instead in exchange for a weakness to Psychic-type moves.
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