1 Lambert: Alvissa, X.
1 Alvissa: Well that’s obvious. Xorn.
2 Mordekai: Objection! I don’t think they exist. Someone probably just made them up to have a monster starting with X.
3 Alvissa: I’m pretty sure they exist.
3 Mordekai: Know anyone who’s seen one?
4 Alvissa: A druid friend—
4 Mordekai: Oh, a druid? What would they know about the natural world?!

According to the Dungeons & Dragons Monster Manual (various editions), xorn are beings with trilateral symmetry, native to the Elemental Plane of Earth. As such, they can move freely through solid earth. They like to eat gems, so adventurers who come across them can distract and appease them by tossing gemstones. Forgotten Realms Wiki has a description and illustrations. (Click the different numbered tabs above the main picture.)

I am 99% sure that Gary Gygax made up the xorn primarily to have a monster starting with X in the original Monster Manual.