Gruclast is a bug Pokémon, although not expressly based on any insect in particular the line as a whole is based on a Fly, the Pokémon as such bears some similarity to the maggot although has a body that is crystal-like in appearance being particularly shiny and having a blue hue to its appearance. It has two very large eyes with pupils that take up nearly 80% of the total eye, it has a very simply looking mouth similar to that of the Pokémon, Caterpie. It has a segmented body that is comprised of six segments including the head, the second, third and fourth segments are the largest and form a hump that the creature has. The Pokémon has six legs located on these three segments which form somewhat of an upside-down "Y" shape similar to a chameleon's hand, the Pokemon can often be seen using the front most feet like hands gripping onto things although is able to climb walls thanks to its feet which have small hooked pads.
The Pokémon's body has an array of spots on each segment, the spots become larger the closer to the top of the Pokémon they are and disappear entirely below the horizontal apex of each segment, the spots are lighter shades of blue and in bright sunlight become invisible because the Pokémon's entire body becomes brighter. Gruclast's fifth and sixth segments act akin to a tail with it able to wiggle them and can even slam them into the ground giving the Pokémon a remarkable jumping ability, the sixth segment splits at the end forming a club-like shape, five protrusions split from the tail forming a cross-shape with the fifth protrusion appearing on the very tip of the tail, it can use this club-like feature to whack would be predators from behind.