The thing is, Keldeo is undoubtedly going to have a gigantic impact on the metagame. All of its primary checks are offensive, bar Jellicent who eats a super effective Hidden Power. Keldeo can afford to run something that's not HP Ice because Hydro Pump is just INSANELY powerful. I'm talking about a 2HKO on offensive Dragonite, through Multiscale. I'll let the implications of that sink in. Personally, the only reliable way to beat this thing is through revenge killing it with powerful physical attackers, ala Scarf Terrakion and possibly things like Raikou in the rain if its not a CM variant.
edit: btw, this is the list of reliable counters for Keldeo:
Slowbro
Slowking
Jellicent
Celebi
Starmie
Latios
Latias
Toxicroak
Amoongus (Only good in DW)
Tornadus
Venusaur
Tangrowth
Ditto (DW)
The pokemon above are really the only things that can switch into Keldeo most of the time, since it's gonna be spamming Hydro Pump more often than not. Everything else on Katakiri's list is handily 2HKOed by specs Hydro Pump.
(btw this is coming from someone who's been playing DW OU for a bit now)
Specs Keldeo in rain is pretty underrated in what it can do; Hydro Pump did 60% to my 220 HP Celebi in Rain before, and I imagine it would do the same to most of the grass types on the list, as well as Slowbro and Starmie. Latias is easily the best counter there, though, given that it beats both of Keldeo's best sets in Specs and CM.











