They can be dual-typed, but at least one of those has to match the type you choose. What type are you choosing, and what’s your team look like?
I would have to choose Bug-type, of course. Team composition would be Galvantula, Yanmega, Heracross, Centiskorch, Genesect, and Rabsca. As to why… I just like 'em, lol. I’m not much of a competitive player. What would your team/type be?
Strong choices all around. Dark type is definitely in my top five, and I love Mightyena. I think it’s easy to sleep on, since you get access to Poochyena literally right out of the gate, but it’s a seriously tough Pokémon.