Ferrothorn @ Leftovers
Ability: Iron Barbs
EVs: 252 HP / 4 Def / 252 SpD
- Leech Seed
- Power Whip
Kyogre and Necrozma-Dusk-Mane make for a solid physically defensive core but are extremely vulnerable against powerful special attackers. To this core, the most threatening special attackers are Kyogre, Xerneas, Eternatus, and Yveltal. Ferrothorn fits perfectly on this team as it is one of the most reliable checks to Kyogre, improves the Xerneas matchup as a soft check, and presents the threat of hazard stacking.
Necrozma-Dusk-Mane already checks Eternatus with Thunder Wave which mitigates the threat by crippling its speed tier; furthermore, it can soft check Xerneas with Dynamax and Max Steelspike. Ferrothorn fits perfectly with these two mons as it improves the Xerneas matchup a back-up option for soft checking in case Necrozma-Dusk-Mane gets chipped, which occurs frequently when matched up against a Zacian-C and Xerneas offensive core. Having Ferrothorn's spikes and leech seed work in tandem with Necrozma-Dusk-Mane's stealth rocks whittles the opponent's team down over time. This core would especially benefit from long-term whittling and trading hazards as it has the capability of winning through longevity thanks to its status absorber in Rest Zygarde and multiple mons with rocks resistant typing in Necrozma-Dusk-Mane and Ferrothorn.
An issue that you would get with running another Kyogre check such as Chansey is the problem of an opponent's Ferrothorn. Ferrothorn easily switches into and hard walls Necrozma-Dusk-Mane and Chansey. With a Ferrothorn of your own, you would gain the ability to deny an opponent's Ferrothorn from recovering HP through leech seed and the ability to trade spikes on the field instead of just having them lay spikes on your side for free. Eternatus is less than optimal on a team such as this because it would have to run a specially defensive set for Kyogre. Specially defensive Eternatus sets are good because they provide toxic spikes support for the rest of your team; however, the last thing you want for a Zygarde that sets up on a trapped enemy Pokémon is for that enemy Pokémon to die on its own before Zygarde can fully set up due to your toxic spikes.