Koin 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):

Species Origin Kremling
Alignment Bad
Kremling Krew
King K. Rool

Koins are Kremlings that wear buckets as helmets and wield shields, appearing as enemies in the Donkey Kong series. They are members of the Kremling Krew, making their debut in Donkey Kong Country 3: Dixie Kong's Double Trouble as among the most common enemies in the game, with one being found in each level. When one is encountered, it will hold up its shield, which protects it from any attacks from Dixie Kong and Kiddy Kong. Furthermore, the Kongs cannot jump on a Koin, but rather use them as a platform thanks to their shield. In order to defeat them, the Kongs must throw a projectile, ideally a Steel Keg, at the back of the Koin where it's vulnerable. Once it is defeated, the Kongs can collect the DK Coin it drops. This must be done to collect every level's DK Coin.

A rarer, pink variant called Koindozers are also encountered in the game. Rather than staying stationary, these enemies will try to rush down the Kongs and push them away with their shield. Like Koins, they can be defeated by throwing a projectile at their backside, and can also be stood on as platforms.