Dark colours, sharp and rough edges all around. Just juxtapose it to the polar opposites, meaning: Dark coloured and sharpedged <> Light coloured and round.
See how that is quite clear what is 'menacing', opposed to 'cute'? If you attach those characteristics to pokemon that they would fit, you might end up with: Deino <> Jigglypuff. And Jiggles is the complete opposite of menacing, it's a rubber balloon.. Oh, unless you're afraid of big round gazing eyes.. 0.0
I still think he's adorable. And I never said that that Jigglypuff was menacing. I don't really find many Pokemon menacing. God forbid I think the blind little dinosaur is cute