Shuckle is mainly based off a turtle but here is some more info
Shuckle seems to be based on an endolith, an organism that lives inside porous rocks or animal shells or possibly an abalone. Fungi, also known as 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.