Well, he can't have those legendaries. Charizards can look vicious look at Kiawe's in the [i]Sun/Moon[i] anime. Also of note, Liam's Charmelon constantly has an angry look on his face, although that stems from things in his past, and his stroy is coming to a head soon. As for Houndoom and Mawile, they would work. Scizor can also look vicious. There is also always describing the Pokémon has having some sort of scar to make it look more 'evil'.