Is it Onix/Steelix?
EDIT: eh, why not.
A shell conceals what might not be - Steelix would be Onix coated in metal, but you don't know.
It’s got so hard you can’t press me - Steelix is hard and has a high defense.
My toothy brother’s thin and long - Steelix has teeth and can use Elemental Fang attacks.
The clawed one’s got hate for him strong - I don't know who that would be, Excadrill?
I am a hardy, poky pal - I don't know.
If you call me, you can call me Al - Al=Aluminium.