Fishin' Boo did not originate from fanon, and more information can be found on their respective wiki(s). You may read more about it on the following wiki(s):

Fishin' Boo
Fishin' Boo SMR
A Fishin' Boo
Species Origin Boo, Lakitu
Alignment Bad

Fishin' Boos, also spelled Fishing Boos, are Boos that resemble Fishin' Lakitus that make their debut in Super Mario World. They carry a fishing rod and and ride a cloud, much like their Koopa counterparts. However, whereas Fishin' Lakitus normally carry 1-Up Mushrooms and other helpful items on their fishing rob, Fishin' Boos carry blue fireballs that will damage Mario when he touches them. In addition, like Boos they cannot be defeated by any means, making them more of a threat. They will follow Mario, carrying the fireball and trying to hit him with it.

Fishin' Boos later appear in Super Princess Peach, where a variant called Calm Fishin' Boos appear. They are first found sleeping, but if Peach wakes them up, they will begin chasing her like a normal Fishin' Boo.