Meganium is a quadrupedal, pale-green Pokémon that resembles a sauropoddinosaur. It has a long neck; yellow eyes; a short, blunt snout; and two stamen-like antennae on top of its snout. The antennae are shorter on a female than on a male. Encircling Meganium's neck are large, pink flower petals with a yellow central pattern and white edges. It has three claws on each foot and a stubby tail.
Meganium's petals release an aroma that calms aggressive feelings and its breath has the power to revive dead grass and plants. Its soothing, regenerative powers can even affect those who stand nearby. Meganium tends to be very docile and has been portrayed as a peacemaker in the anime. It can rarely be found living in grasslands.
Meganium has the appearance of a sauropod, particularly an Apatosaurus. Its flower seems to be based on a type of hybrid daylily with ruffled edges and its long neck makes it similar to the titan arum. It may also draw inspiration from geraniums, as suggested by its name. Meganium's design may have also been influenced by Aachenosaurus, an ancient plant whose fossils were once believed to belong to a dinosaur.