It is kind of hard to tell bet once you see it you will understand.
Shelmet is a knight's helmet combined with a snail.
Karrablast is based off of the Japanese Snail-eating Beetle.
Accelgor is rather hard to explain but Bulbapedia explains it perfectly.
>Accelgor appears to be based on an auto racer or a ninja, combined with a giant tube worm. Its cloth-like "clothing" and Bug type suggest it may also be based on the bagworm pupa.
Escavalier is also based off of the Japanese Snail-eating Beetle but it is combined with a jousting knight.
Hope I helped!
(Sorry for the big pictures.)