Roundworm

The interior of Majin Buu's body is not empty space. It is a bizarre, living ecosystem, and among its inhabitants are the Roundworms, known in Japanese as Kaichu Oyako. These creatures are large worm-like beings that have adapted to survive within Buu's internal environment, feeding and reproducing in a landscape made of the pink monster's own biological material.

Their size is considerable, far exceeding what anyone would expect from a typical parasitic worm. They exist in multiple life stages, with larval forms appearing alongside fully grown adults. The fact that Buu's body supports an entire self-sustaining population of these creatures speaks to just how alien and vast his internal biology truly is.

Goku and Vegeta encountered the Roundworms during the Fusion Saga, after allowing themselves to be absorbed by Super Buu in order to rescue Gohan, Goten, Trunks, and Piccolo from within his body. Navigating Buu's interior was already disorienting enough without the added threat of hostile creatures, but the Roundworms served as yet another obstacle in an environment that seemed designed to kill intruders.

The father Roundworm and its offspring proved to be aggressive defenders of their territory, attacking the two Saiyans as they searched for their friends. While hardly a serious threat to warriors of Goku and Vegeta's caliber, the Roundworms added to the surreal danger of the situation and reinforced the idea that being inside Buu was itself a form of combat.

In Dragon Ball Z: Attack of the Saiyans and other video game adaptations, the Roundworm family is expanded into multiple variants. Standard Roundworms and their Larvae appear alongside more exotic types like Dark Fireflies with their Firefly Larvae and Poison Sandbugs with Sandbug Larvae, creating a full bestiary of creatures that inhabit Buu's internal world.

The Roundworms are one of the more unusual "races" in the Dragon Ball universe, existing not on a planet or in space but inside another living being. They represent the creative extremes Toriyama was willing to explore during the Buu Saga, where the concept of what constituted a battlefield expanded beyond any previous definition to include the literal insides of a magical creature.