If I slash the ones that are shit mons you are left with three.
Also you forgot Victini and Mega Lucario yet you listed Venusaur and Arceus-Fire.
For the sake of argument, Victini requires sun support to beat Xerneas and Mega Lucario fails to KO with Bullet Punch. On the other hand, Mega Venusaur and Arceus-Fire are able to Roar it out, as can Arceus-Steel, too. This is indeed a valid argument in stopping Xerneas as once you pseudo-haze a Xerneas after it has consumed its Power Herb, it is much less of a threat. Chaney and Blissey are decent checks when Xerneas lacks Substitute or Close Combat, Excadrill beats most variants in sand, and Genesect beats most variants with Sticky Web support. I kind of assumed that you knew what I meant when I said that the Pokémon I listed are able to check Xerneas; adaptation to the nature of the team structure was insinuated but I wasn't clear enough, my bad.
Yeah what minority slashed aren't checks. And you missed hippodown, lando-t, heck even salamence, from ekiller's checks that are viable in the current meta (lando is a shitty check but it is a check), and stalltwo; giratina-A is terrible as is skarmory (fire coverage more common than ghost these days on ekiller it's just not really used) and arc-fighting; if you do include skarmory as a check then ferrothorn, a more viable pokemon in this meta needs mentioning too, whilst support arceus forms with WoW (u mentioned ghost, but rock and water are valid too) are notable checks. So yeah ur list is kinda bad =/ Point thus not made.
I did miss Landorus-T, yes, as it can switch in on the Swords Dance, stomach an Extreme Speed, and retaliate significantly with a Superpower or Earthquake. On the other hand, Hippowdon and Salamance are not checks because while it is able to stomach a hit and pseudo-haze Arceus out, it cannot mar the Alpha Pokémon substantially enough to pressure it; this means that it can easily find another opportunity to set-up with Swords Dance and threaten your team. Now, some may claim that this is an invalid claim because I claimed that Skarmory is a counter, too, but there is a difference of central importance when comparing Pokémon like Skarmory to other supposed checks like Hippowdon and Salamance: and that is Skarmory's ability to wall Ekiller indefinitely.
+2 252 Atk Life Orb Arceus Shadow Claw vs. 248 HP / 252+ Def Skarmory: 107-126 (32.1 - 37.8%) -- 94.2% chance to 3HKO
+2 252 Atk Life Orb Arceus Extreme Speed vs. 248 HP / 252+ Def Hippowdon: 207-243 (49.4 - 57.9%) -- 60.5% chance to 2HKO after Leftovers recovery
+2 252 Atk Life Orb Arceus Extreme Speed vs. 248 HP / 252+ Def Salamence: 266-316 (67.6 - 80.4%) -- guaranteed 2HKO
As you can see, the difference is quite noticeable. While Skarmory is able to Roost and Whirlwind without worry, losing about 1/3 of its health to Shadow Claw considering the following Leftovers recovery, while Hippowdon and Salamanca are in fact threatened to a 2HKO. It is also worth noting that, while Shadow Claw is indeed has a high critical hit ratio, critical hits are x1.5 instead of the x2.0 last generation.
Ferrothorn is an interesting check as it can sap Arceus's HP with the combination of Leech Seed and Protect, but once you switch in as it Swords Dances, you need one more turn to Leech Seed it. As the opposing Arceus Swords Dances again, it is at +4 with you having landed some Leech Seeds. Finally, you Gyro Ball as it boosts to +6 and it will already be too late and you would have a bit more than a 50% chance to hope for you not being OHKOed: +6 252 Atk Life Orb Arceus Earthquake vs. 248 HP / 252+ Def Ferrothorn: 317-374 (90.3 - 106.5%) -- 43.8% chance to OHKO. At least, that how I view the scenario when considering Ferrothorn as an Ekiller check, and as you can see from the situation I illustrated, it's shaky at best.
I also do not consider StallTwo and support Arceus to be checks for two reasons: first, with Life Orb, both are KOed after a +2 hit without burn and a +2 hit with a burn. Second, Arceus may opt to carry a Lum Berry, resulting in any effort to bring in a bulky Pokémon to burn it futile. =/ counter-argumentative point thus not made.
In summary, as a revisal to my original claim, only one Pokémon was added to the list of Arceus-Normal's checks: 1, and none of Xerneas's checks were removed. Despite attempts of refutal to my stance on this, I still believe Arceus-Normal easily deserves a spot in the S Rank, trampling all other candidates to it with its mighty horse legs.