The Mingo Kids are minor antagonists that appear in Hard Luck and The Meltdown. They're a group of kids (presumably relatives such as brothers and/or cousins) who appear to be quite feral--according to Greg, he's never seen any of the Mingo-kids enrolled in school at any point and as far as he or anybody else knows, the Mingo-kids just live in the woods like a pack of wild animals. The leader of the Mingo-clan is some kid named Meckley, who always wears a tank top and a belt with a gigantic metal buckle on it. It's unknown who the kids' parents are (or if there are even any adults in the Mingo-clan).
The Mingo-kids often chase/threaten people who happen to walk by their woods (typically kids who're walking home from school). And at the end of Hard Luck, Meckley (along with some other members of the clan) chases after Greg and Rowley with his belt.
The Mingos are first mentioned in The Meltdown when Greg says that nobody sees any of the Mingo-kids during the winter, and by the time they show once winter's over and spring starts, they look like they just woke up from a long nap. Greg says that if the Mingos don't hibernate (or something like that), nobody knows what they do during the winter. The Mingos are mentioned again when Greg and Rowley accidentally stumble upon their camp in the middle of the woods (when they're trying to get away from some Whirley Street-kids), where Greg finds Meckley's belt buckle and accidentally leaves his glove/mitten/puppet, Mr. Morsels, behind.
Later on in The Meltdown, the Mingo-kids end up coming out the woods where the kids in Greg's neighborhood are having their big snowball fight/war, where Meckley has Mr. Morsels on the end of a stick and Greg realizes that he has Meckley's belt buckle (Greg figures that he must've put it in his pocket without even realizing it). They're apparently not liked by everyone as when the Mingo Kids attacked, the Hill and Bottom Survey Street Kids, the Safety Patrols, and the Whirley Street Kids joined forces to attack the Mingos.