Yumboes are a kind of African house-elf. They are very spiteful and like to steal food from humans that cross them. They can vanish at will and have intrinsic magical power (Pm).
Very strict security measures were put in place during the 1998 Quidditch World Cup due to the riots that happened the night of the final of the previous World Cup. One incident was the arrest of Senegalese team mascots, Yumboes, outside the stadium. The Yumboes took their revenge by stealing all the food in a ten-mile radius and vanishing into the night (Pm).
Yumboes are a type of fairy from the mythology of the Lebou people who lived in what is today Senegal. One of the traits attributed to them is the theft of corn from humans.
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"Yumbo" on the Harry Potter Wiki