Big Green Caterpillar

Worms are long, thin, tube-shaped creatures that inhabit both Mushroom World and Earth. They often appear as either enemies, characters or gameplay elements.

The Super Mario Bros. Super Show!
A single worm first appeared in the The Super Mario Bros. Super Show! episode "The Bird! The Bird!". After the Mario bros. defeated Bowser and saved their new friend Cheepy and his mother, they were invited over to Cheepy's house for dinner as thanks for their help, however they were unaware that the dinner was actually a large, talking, grumpy worm, after seeing it they ran out of the house.

Super Mario Bros. film
In the Super Mario Bros. film, worms were used as sustenance and drink toppings for the people of Dinohatten. Lena, one of the main antagonists of the film was seen drinking a worm as it screamed for mercy, showing that the worms of Dinohatten are indeed intelligent.

Wario Land 3
In Wario Land 3, a worm named Wormwould appeared as the boss of The Grasslands and Wario had to defeat him in order to gain the Music Box that opened the way to The Big Bridge. In the game there was also a worm-like enemy called Silky that could turn Wario into Ball o' String Wario.

WarioWare, Inc.: Mega Microgame$!
In WarioWare, Inc.: Mega Microgame$!, one of the playable microgames in the Nature folder is known as Worm Squirm where the player has to find out which worm is the one under the player's control and guide it towards the water.

Super Mario Galaxy
In Super Mario Galaxy, a giant, friendly, green worm appeared in the Gusty Garden Galaxy, where it was found snacking on some giant apple planetoids. Mario could use this worm to cross between the three apple planetoids in the area by Ground Pounding Stumps in the planetoids, causing the worm to burrow through one apple and into the next.