>Shuckle seems to be based on an endolith, an organism that lives inside porous rocks or animal shells or possibly an abalone. Fungi, particularly mold, are common endoliths; this explains why Shuckle is the mold Pokémon. It may also be based on scale insects – small, shelled parasites that produce a sweet fluid called honeydew that attracts other insects. Shuckle may have been based on a variety of mollusks, as it shares some characteristics with them. Some examples of the similar characteristics are the shell, muscular and limp-seeming feet, the use of acids to receive food or to break down obstacles, likes fermented foods, and both also live close to the water. It may also be based on turtles or tortoises, appearance-wise.
That pretty much sums it up for what it is supposed to be.
Now, as for it's typing, it is probably Bug type because Bug type is the closest Pokemon type you can get to mold. Rock type because it is/lives in a rock. Or, just GameFreak logic.
Hope this helps!