You’re kidding! Gray has one too!

A new feature in X and Y is that male parents have a 20% chance of passing their hidden ability to a child. But because the offspring is always the same species as the female, the two pokemon have to be the same thing. In short, you’d have to have a second Froakie/Frogadier/Greninja that’s female (themselves having a low chance of being born, which is supposed to play into Pokemon canon of why the starters live on a nature preserve and not in the wild) and after that play against a 20% chance of the hidden being passed on.

On the other hoof-! If the Froakie you got is already female (with X and Y marking the first time that female starters can have their hidden due to previous limited distribution), then you’d just need to breed that a few times to get a Protean Froakie, since females have a much larger 60% chance of passing on their hidden! And if both male and female have their hidden the chance jumps to like 80% or higher!

… I probably know too much about this stuff.

Anyway, if you don’t already have a female, it would probably be easier for Fuse to trade. But I can still get you both an Iron Fist Chimchar with Thunder Punch! Very powerful counter against water-types!