The xan, x, is a monster in NetHack. Its unique attack targets your legs, resulting in a wounded leg if the attack succeeds and you are not riding, flying, levitating, or wearing boots (iron shoes and low boots protect you 40% of the time, other boots 80%). It can also walk on water. Its corpse is poisonous if eaten. If canceled, the xan can no longer injure your legs, and instead will "nuzzle" them.
In SLASH'EM, the xan can't reach your boots if you are flying.
The monster 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.