From the Super Mario Wiki
A Slug is an animal in the Real World and a colorful creature that appears in Yoshi's Story. Slugs look like they are made out of slime or Graffiti. They also have two bulging eyes. In Page 2: Cavern, Yoshi can go into a pot and enter a room with a few Slugs crawling up, down and horizontally on the walls. Slugs also appear in Stage 6-4: Magma Castle.
It is difficult to defeat Slugs in combat. Many eggs are required to destroy a single Slug, as they must first be stunned by eggs exploding in front of their eyes before they can be damaged. However, once Yoshi does destroy the Slug, he will get a special prize.
Slugs are one of the few mini-bosses in the game.
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