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. 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, and a calmer variety called Calm Fishin' Boo also appears. Both types reside in Shriek Mansion on Vibe Island, and only Rage can defeat them.
They were also slated to appear in New Super Mario Bros. Wii, as an unknown object was named similar to Fishin' Boo.
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.
Names in Other Languages
||Pun on Boo and Lakitu and the word "Pescatore" ("Fisherman")