Although Fishin' Boo is called a Boo, it bears little resemblance to other species of Boo. Instead, it appears to be a ghostly version of Fishin' Lakitu. It flies near the top of the screen in a cloud, (unlike Lakitu, the Fishin' Boo's cloud has no smiley face) following Mario wherever he goes. It holds a fishing pole, with its bait dangling right at Mario's jumping height. Whereas the Fishin' Lakitu holds a desirable 1-Up Mushroom, the Fishin' Boo dangles a deadly blue flame. Mario is safe so long as he runs underneath the flame, but it serves to make jumping supremely hazardous. In the original game, Fishin' Boo appears in the Chocolate Ghost House, where moving holes and Eeries at floor level force him to jump often.
Some people call this enemy "Laki-Boo" because of its resemblance to Lakitu. This sprite can ratchet up the difficulty of your levels in a hurry, and since the vast majority of hackers either put it at a very low height or in a level built with lots of slopes, its use is discouraged in your hack except at levels late in the game.
Fishin' Boo is sprite AE. The Boo itself uses palette E, its pole uses red palette C, and its flame uses blue palette B. (The palettes of its individual parts can be changed with a hex editor) It will only appear properly with the Ghost House sprite set. (SP3=06, SP4=11)
You need to use Sprite D2 (Turn off Generator 2) to make it go away. If Fishin' Boo is killed with a star, its graphics will glitch. This can be fixed by setting it to "Disappear in a cloud of smoke" with Tweaker.
There are also boss versions of this sprite available in SMW Central's Custom Sprites section.