From the Super Mario Wiki
Fishin' Boos are the spirits of Fishin' Lakitu found in Super Mario World and Super Princess Peach. They carry a fishing rod and ride a Lakitu's Cloud, much like the living variety. However, instead of a 1-Up Mushroom, Fishin' Boos carry a blue flame and follow Mario or Luigi in a way that makes it hard for them to Jump. Their clouds also do not have eyes. A Fishin' Boo has the same immunities as a normal Boo and therefore cannot be jumped on.
Fishin' Boos also make an appearance in Super Princess Peach as Fishing Boos, and a calmer variety called C. Fishing Boo also appears. Both types reside in Shriek Mansion on Vibe Island, and only Rage can defeat them.
A "Fishin' Boo" appears in Mario & Luigi: Bowser's Inside Story in Bowser's level up screen. However, it was actually a Boo with a fishing rod and not an undead Lakitu.
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