The xan, x, is a monster in NetHack. Its unique attack targets your legs, resulting in a wounded leg if the attack succeeds. Riding, flying, or levitating prevent a xan from wounding your legs. Worn iron shoes or low boots protect you 40% of the time; any other boots protect you 80% of the time. It can also walk on water. Its corpse is poisonous if eaten. A canceled xan cannot injure your legs, and instead "nuzzles" them.
The xan is based on a mythical Mayan mosquito from Popol Vuh, a Mayan book.
They sent their friend the mosquito [xan] ahead of them to find out what lay ahead. "Since you are the one who sucks the blood of men walking along paths," they told the mosquito, "go and sting the men of Xibalba." The mosquito flew down the dark road to the Underworld. Entering the house of the Lords of Death, he stung the first person that he saw...
The mosquito stung this man as well, and when he yelled, the man next to him asked, "Gathered Blood, what's wrong?" So he flew along the row stinging all the seated men until he knew the names of all twelve.