The thing they all have in common is being awesome.
I find pure poison types tend to look either sly (Arbok, Toxicroak ect.) or monstrous (Weezing, Muk, Garbodor ect.). Part poison types however aren't so easy to relate. Anyone want to try to find a real similarity of Bulbasaur to Gastly?
Ekans is plotting something .... Here's a drawing I did at 4 am in the morning...
i think its hillariouslly bad.