Gnawty 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):

Very gnawty 01
Very Gnawty, a notable Gnawty.
First Appearance Donkey Kong Country (1994)
Kremling Krew

Gnawties, also called Beavers, are lazy-eyed beavers and basic enemies that appear in the Donkey Kong series, debuting in Donkey Kong Country. They come in two varieties; green and brown. Green Gnawties are the most common, and will walk around back and forth. They can be defeated by almost every attack. The brown variants are a lot less common, and will ride on Millstones that cover a large area to try and damage the Kongs. They are invincible and cannot be defeated.

Fanon Apperances

Super Mario Spikers

Gnawties appear in Super Mario Spikers, appearing as Balanced-type Sidekicks. They wear a jersey in this game, which represents their team, having the team emblem at the front of it. The Gnawty's skin color and jersey color will also change depending on the team. Their theme is a faced-paced remix of the Treetop Rock theme from Donkey Kong Country.