Just a point, any chance that Crobat could work as a check? It switches into PuP and takes an absolute pittance, then either throws out Haze on a predicted Sucker Punch (or regardless) or uses a solid attacking move to outspeed a non-priority option. Although I'm not sure how hard it can hit and if it doesn't Haze the boost off then it's pretty much dead so it's a backup strategy at best. Plus it handles a switch out very poorly.
Kind of a pity Malamar can't get fast enough to outspeed it, really. Topsy-Turvy plus STAB Superpower would be a very convenient way to deal with it. As stupid as it sounds, Scarf Malamar might be able to deal with Mega Kanga if it switches into PuP: It beats anything that isn't Sucker Punch by simply Superpowering it into oblivion, and beats Sucker Punch with Topsy-Turvy (allowing another pokemon to switch in and take negligible damage from -2 PuP at which point it's facing +0 Mega Kanga, which is far less threatening). Additionally, if Mega Kanga switches out, the incoming Pokemon will be hit by Superpower and Malamar nets a free Bulk Up which severely limits what switch-ins will actually be safe (something like Skarmory, for instance, will have to cop two Superpowers before phazing which will put a fair bit of pressure on it).
I guess the general point here is that since Kangaskhan's main form of priority is Sucker Punch, something that can come in on PuP and threaten to erase boosts on a predicted sucker punch (via hazing or phazing, but not Clear Smog, Dragon Tail, or Circle Throw) or outspeed and cripple or OHKO has the potential to threaten it. Gengar, for instance, can win with successful prediction by Hazing Sucker Punch, then using Substitute if another Sucker Punch is predicted which ensures that even a third Sucker Punch will only hit Gengar at 40 BP, while it retaliates with a Focus Blast. If Crunch is predicted instead, Gengar can simply outspeed and Focus Blast straight off. Furthermore, it takes NVE damage from PuP on the switch thanks to a secondary Poison typing, further helping it win the matchup.
Other options include:
- Aerodactyl, mega or no (can Whirlwind it out on a predicted Sucker Punch and resists Normal STAB, although it does worse against Crunch variants)
- Manectric (Intimidate already removes a boost and it packs quite a wallop as well as access to Roar, although without an OHKO it will likely die)
- Garchomp (Rough Skin/Rocky Helmet allows it to score a likely KO with an attacking move, Roar lets it shoo out Sucker Punch attempts, Focus Sash would allow it to tank the Punch and KO back, while using the Mega would enable a first turn outspeeding due to the mechanics of it, followed by either Roar or a powerful attack like Outrage)
- Zoroark (Takes low damage from a Sucker Punch, can Roar it out on Sucker Punch, outspeeds and Focus Blasts it, can trick it into thinking it doesn't outspeed. Primarily a revenge killer due to being unable to switch in and not be revealed, plus taking high damage from PuP, but it can potentially trick Kanga into staying in with Illusion)
- Volcarona (Threatens Whirlwind or Quiver Dance on a Sucker Punch, tanks PuP without problems, deals heavy damage although on a speed tie unless something intervenes)
- Noivern (Whirlwinds Sucker Punch, Focus Blasts non-Sucker Punch, tanks PuP. Pretty standard)
- The Sableye/Cofagrigus checks mentioned above fit in well here
It's important to note that if you lose a Pokemon but significantly damage Mega Kanga, Extremespeed, faster priority (Talonflame, for instance), or strong Fighting-type Priority (Breloom, Lucario, et cetera) can finish it off when it shows up again. Obviously having to lose one Pokemon isn't ideal, but that still means one "check" and one powerful priority user. Ultimately this is all theorymon and I know prediction is unreliable, but I think some of these Pokemon at least give you a decent shot against Mega Kanga rather than just going immediately splat. Don't forget that if you can get a Reflect up using the same Sucker Punch trick you also end up with additional options.