Those are amusing. =)
Truant works better than Rocky Helmet though. Mega Kangaskhan always carries Earthquake, which doesn't make contact, so Iron Barbs+Rocky Helmet doesn't activate, which is what Mega Kanga will do if it sees Ferrothorn. +2 Earthquake is an easy 2HKO on Ferro, and Mega Kanga can survive the damage from 1 IB+RH and a Power Whip/Gyro Ball, allowing it to set up the Power Up Punches. Despite that, Ferro can survive the next attack and KO Kanga. So the issue here is that Rocky Helmet Ferro is more of a check, not a counter, since for it to win it has to come out the same time as Kanga. If it switches into a PUP, it'll get killed before it can kill Kanga.
As for Truant Durant, it's also a check. +2 Sucker Punches and Earthquakes kill it (though with defensive investment it has a chance to live). So it also can't switch in on a boosted Kanga. It can switch in on an unboosted one, live through a Sucker Punch, and then use Entrainment to cripple Kanga. If they are sent out at the same time, Durant easily wins.
The problem people generally have with Mega Kanga is that its really difficult to counter; borderline impossible because of Parental Bond. But it is a lot easier to check.