There is not much reason to keep at least one ability, in this case Prankster, consistent upon evolution. Why not have the best of both worlds and have the abilities be Rattled / Scrappy? Either way, there is some believable development in personality upon evolution (which I think many of us are implying). The pre-evo could start out a scaredy-cat (no pun intended, but lol) and conquer its fears so that it is a fearless, intimidating presence. On the other hand, we could think of the pre-evo as having a scrappy personality, being a fierce fighter, though an inexperienced one. It grows in knowledge and wisdom, and learns to control these tendencies as a sophisticated prankster.
Rattled could be interesting, but the flavor of the ability implies that the Pokemon is scared of or at least unnerved by Bug-, Ghost-, and Dark-type moves. As the pre-evo is Normal / Fighting, and therefore resists two of those types and is immune to the third, that sounds a bit... odd.
I'm going with Scrappy/Prankster. Many Pokemon gain bravery upon evolving, but unless they're a total wimp in their first stage they probably won't have Rattled. I think this preevo is definitely not a wimp right now, just not the bravest.
I personally like Hustle/Prankster orRattled/Prankster for reasons already stated.
Hustle, cubs play-fighting with each other grow up to become 'Intimidating' warriors.
But then I also like the sound of Rattled. A cub overcomes its fears after evolution to instead be... Feared, I guess.
I'm pretty sure this isn't a bold vote just yet people.
I, too, throw my support around Scrappy/Prankster just because that's how I consider baby lion cubs to act, always pulling fast ones on their siblings. Also, the dictionary definition of scrappy includes: "Determined, argumentative, or pugnacious," which all lend themselves to the Fighting type really well. And it does have excellent competitive prospects in unresisted STAB's, even though that's not really an issue.
For Rattled, I don't see this pre-evo as a scared little wimp. It's not a full refined fighter yet, but it can still battle. I think Scrappy shows this really well, whereas Rattled does not. And how many pranksters have you seen that are easily scared? Usually they're confident, ready for a Scrappy fight. Also, Rattled is absolutely pointless, as it resists Bug and Dark and is immune to Ghost.