Inside Shelgon’s armor-like shell, cells are in the midst of transformation to create an entirely new body. This Pokémon’s shell is extremely heavy, making its movements sluggish.
– Ash’s Pokedex
Shelgon is one of the few pure Dragons (before Gen V, when the added 4 pure Dragons). It’s in the pupa stage, like many mid-stage evolution Pokemon (Metapod, Kakuna, Pupitar, Cascoon, Silcoon), before it becomes fully grown. Shelgon is hiding in its shell, its eyes looking out warily.
In the anime, there was an episode featuring a Shelgon that wanted to fly. Even though it doesn’t have wings, it would keep on trying to fly. In the end, it finally evolved and thus was able to fly.
Stat-wise, it has good Defense and Attack, but is pretty slow. It can learn Dragon Claw by level-up, despite not having visible claws. Its feet are outside of its shell I suppose, but the shell on its body looks too heavy for it to be able to stand up and use its legs to claw. Its mouth is hidden in its shell, but it can learn Bite and Crunch.