Wormy is one of Sandy's pets who only appears in the episode of the same name. In the aforementioned episode, he matures from a monarch caterpillar into a monarch butterfly.
As a caterpillar, Wormy is a light shade of green with black eyes and white spots.
As a butterfly, Wormy is dark orange with black stripes and black on the outermost parts of his wings; this design is that of a monarch butterfly.
At some point, Sandy bought or found him in Texas. Sometime later, she took Wormy to her home in Bikini Bottom.
He later turns into a butterfly overnight.
When SpongeBob and Patrick are pet sitting for Sandy, they and wormy play games such as hide-and-seek and tag. However, over night, Wormy metamorphoses into a butterfly and scares everyone in Bikini Bottom because they've never seen a butterfly before.
In The SpongeBob Movie: Sponge Out of Water, a poster of Wormy in his butterfly form can briefly be seen in Sandy's treedome. The image of Wormy on the poster looks very similar to the one seen on the "Beware" fliers made by SpongeBob and Patrick.
The close-up shot of Wormy in his butterfly form is actually footage of a buzzing horsefly.
During the news bulletin about the "monster," the live-action monarch butterfly stock footage depicts a female butterfly (hence the thicker wing veins than the male), suggesting Wormy may have actually been female.
The episode "Wormy" was banned in some countries because of Wormy's disturbing close-up shots.