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.