Judd did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
“This is Judd. He’s a cat, if you can’t tell. Long shrouded in mystery, details about Judd are just now beginning to trickle in. Firstly, we can confirm that he’s (probably) not a dog. Also, Judd was born with the incredible ability to judge turf area at a glance to separate the winners from the losers. And it may look like he’s wearing a snazzy cat tuxedo, but that’s actually just the naturally occuring pattern of his fur. Some cats have all the luck.”
— Splatoon Official tumblr
Judd is a cat that appears in the Splatoon series, and the only known creature resembling a (land) mammal. Judd acts as the referee in Turf Wars and Splatfests, and has the special ability to see within a single glance which color has the most ink on a turf, which is nifty to determine the winning team on a Turf War.
Judd is kind of lazy, which is to be expected from felines like him. After judging a Turf War, he can be seen sleeping on a trash bin.
In future titles, Judd is accompanied by a kitten that looks similar to him, named Li'l Judd.
Judd is a plump feline with short legs. His fur is predominantly with black spots and his eyes are yellow. His abdomen has a peculiar fur pattern, resembling a dapper tuxedo with a bow tie. Judd's tail is also shaped like an arrow, which he can conveniently use to point towards the winning team with it.