From the Super Mario Wiki
Piro Dangles are enemies from Super Mario World 2: Yoshi's Island and its remake Yoshi's Island: Super Mario Advance 3. They are invincible wisps of fire. Piro Dangles can only be found in a secret room in Burt the Bashful's Fort. These fire creatures simply travel around large blocks, and will hurt Yellow Yoshi if he touches one of them. The Piro Dangle looks very similar to another enemy from the Mario series - Spark from Super Mario Bros. 2. In Super Mario Advance 3: Yoshi's Island, they reappear in Endless World of Yoshis, and can now be obliterated with a Special Blue Watermelon. Their faces seem similar to that of Fryguy.
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