There are not a lot of X words in D&D that come to mind. There is a monster in the Monster Manual called a Xorn. It looks like living rock with three arms and three legs and a mouth on top of its head. It can move through rock and seeks out metal to eat. The same metals adventurers prize, copper, silver, gold, etc. They are very tough and hard to fight. They are of average intelligence, so you can reason/bribe them to leave you alone with the right amount of gold for them to eat.
I have never ran into one as a player or used one as a DM.