Prankster
Except... a lot of legendaries did get Hidden abilities. Mewtwo's Unnerve, Ho-Oh's Regeneration, Lugia's Multiscale, and Dialga, Palkia, and Giratina (both forms!) come to mind. Also, many of the abilities that lack a Dream World ability are in the first group you mentioned - that is, their concept is heavily tied to their ability (Groudon, Kyogre, Rayquaza, Regigigas, Darkrai, Arceus, Reshiram, and Zekrom come to mind.)Flavor Ability
The only real Pokémon with no hidden abilities are stuff like Kecleon and Castform (heaviliy tied to their ability concept-wise), most legendaries and Pokémon with Levitate as their only ability (or evolved from Levitate-only Pokémon like Dusclops/noir), and I recall Starly being the only exception (though not really, since veekun gives Keen Eye both as normal and hidden ability which should be considered separate since Staravia/raptor get Reckess instead of Intimidate). Tomohawk doesn't fit in any of these categories, so I think he should get one.