Aegislash-Hard counter to any set that lacks Toxic.
Bisharp-Hard counter unless flinch
Charizard (Mega-X)-Hard check to DD sets with enough phys def investment; hippo lives the +1 Flare blitz and kills with EQ.
Charizard (Mega-Y)-Hippo serves as a hard check to zard y once it has already mega evolved; it removes the effect of solarbeam and avoids the 2hko from Fire Blast, and can OHKO with Stone Edge/rock slide.
Deoxys-S-Counters LO sets. Life orb ice beam doesn't 2hko, and hippo slacks off the damage while deo s takes continual residual damage.
Thundurus-I-Hard counter, unless thundy has Grass Knot, which is very uncommon.
A Rank: Reserved for Pokemon who can sweep or wall significant portions of the metagame, but require some support or have some flaws that prevents them from doing this consistently. Supporting Pokemon in this rank may give opponents free turns or cannot create free turns easily themselves, but can still do their job most of the time
A+ Rank
Azumarill-no
Garchomp-with a phys def spread, it hard counters all chomp sets that lack SD; hippo is admittedly setup fodder for SD chomp.
Greninja-no
Gyarados (Mega)-no
Keldeo-no
Kyurem-B-no
Landorus-I-no
Mawile (Mega)-Soft Check:Lives the +2 play rough with enough investment and kills with EQ if it has been weakened
Pinsir (Mega)-hard check; hippo lives the +2 frustration and kills with SE
Scizor (Mega)-Hippo is complete setup fodder for this, so i'll say no unless hippo has WW.
Talonflame-Hard counter
Tyranitar (Mega)-HArd counter with physically defensive evs
A Rank
Clefable-Hippo is setup fodder for magic guard clef if it lacks WW, but it wins against Unaware variants due to toxic.
Dragonite-It slacks off damage from +1 outrage, and then 2hkoes with SE.
Excadrill-Hard Counter
Gengar-with Whirlwind or Stone Edge, it can tentatively handle certain gengar sets.
Heatran-hard check unless specs set
Landorus-T-Hard check to any set that lacks DD; hippo toxics and stalls it out
Latios-no, but it can slack off the damage from Draco Meteor and live to tell the tale, but it can't switch in.
Manaphy-no
Rotom-W-no
Terrakion-hard counter unless band, which has a chance to 2hko
Tyranitar-hard counter
A- Rank
Garchomp (Mega)-no
Gyarados-no
Latias-hippo can freely switch into any set that lacks Life Orb Surf
Mamoswine-no
Politoed-no
Scizor-setup fodder for SD sets, but hard counters Band sets
Zapdos-decent check if Zapdos lacks Toxic
B Rank: Reserved for Pokemon who cannot sweep through or wall significant portions of the metagame, but can properly fulfill a given offensive/defensive niche. Support Pokemon in this category have flaws that prevent them from doing their job or are setup bait for dangerous sweepers. Pokemon who are partially outperformed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category.
B+ Rank
Conkeldurr-hard counters AV set with enough physically defensive investment
Diggersby-no
Gardevoir (Mega)-no
Kabutops-no
Medicham (Mega)-no
Scolipede-Hard counters Life Orb sets
B Rank
Blastoise (Mega)-no
Breloom-no
Heracross (Mega)-no
Lucario-Hard counter with physically defensive spread
Kingdra-no
Manectric (Mega)-HArd counter in all scenarios
Raikou-Hard counter in all scenarios
Tornadus-T-hard check unless multiple hurricanes are hit in sand
Victini-serves as a check to scarftini, but cannot switch into bandtini.
B- Rank
Absol (Mega)-hard counter
Alakazam-hard counter to sashed zam, because no move comes close to 2hkoing and hippo EQs in retalition.
Alakazam (Mega)-hard check, because no move outside of energy ball 2hkoes. energy ball isn't common on this in the slightest.
Crawdaunt-no
Entei-hard counter
Gothitelle-no
Rhyperior-hard counter, but no one uses this
Sharpedo-no
Staraptor-no, 2hko from reckless Brave Bird
Starmie-no
Volcarona-Hard check because hippo ohkoes with SE.
Zygarde-Setup fodder unless hippo has WW.