Btw, for people who are bringing up that Rocky Helmet mons can counter MegaKhan, there's this nifty little move called Knock Off that MegaKhan's teammates can use, most of them being unexpected.
So if your "counter" to MegaKhan is Rocky Helmet ferro, chomp, or skarm (and Eviolite Dusclops lol) then you'll have to play around Knock Off too or else MegaKhan is gonna have a field day with your leftover-less Ferrothorn.
You're missing my point, which is to use Stealth Rock early, then keep Ferrothorn off the field until they bring in (Mega) Kangaskhan. You don't expose Ferrothorn to the chance of getting Knock Off-ed, and you keep it healthy enough (45%) to survive one hit from a fresh Mega Kangaskhan. That's literally all you need to do; swap Ferrothorn in the moment Mega Kangaskhan hits the field.
If they predict and swap Mega Kangaskhan out for their Ferrothorn counter, then all that happens is you both swap Pokémon on the same turn. Then you just do what you'd normally do when the opponent brings out a Ferrothorn counter. You swap Ferrothorn out for your answer to it.
Or you just sacrificed FERROTHORN of all pokemon for one of your opponents pokemon... Again, Rocky Helmet Ferrothorn has been brought up at least 70,000 times. It still is not reliable enough, and Rockey Helmet is too gimmicky and not optimal on Ferrothorn in general. So. Yeah.
What value is Ferrothorn after setting up Stealth Rock and/or Spikes? Practically none. It can tank attacks, but it doesn't have enough offensive presence to do much. After hazard setting, you use it to TWave or Leech Seed the opponent, to stop or discourage them from trying to set up. Running this doesn't detract from Ferrothorn's usual performance or roles, it enables it to stop Mega Kangaskhan.
Mind you, I'm still in favour of banning Kangaskhanite, I just want to explain why I think Helmethorn is underrated as a pseudo-counter.