Mawile, the Deceiver Pokémon. Mawile has jaw-like appendages at the ends of its steel horns. It decieves any opponent who lower its guard with its hypnotic expression, then quickly spins around to bite them.
– Ash’s Pokedex
Mawile is the first pure Steel type Pokemon, even though it doesn’t look very steel. It also looks like it should be a female-only Pokemon, but it has equal gender ratio. Its face looks friendly, but the mouth on the back of its head looks vicious.
Up until Gen V, Mawile is the only pure Steel type Pokemon. It has Defense and Attack as its highest stats, even though its total stats aren’t great. It has no evolution. By level-up, it learns more Dark type moves than Steel type moves (it only learns one attacking Steel move by level-up), which, along with its Pokedex entries as the “Deceiver Pokemon”, and its colouring, make me think it should be pure Dark type instead of pure Steel.