Jormungar

A Jormungar in the Borean Tundra

Jormungar concept art

Same art, colored

Jormungar are creatures that can be encountered in Northrend, via the WotLK expansion. They have flattened bodies with rows of muscular crystalline spines along either flank.

From the official  World of Warcraft Community Site Wrath of the Lich King — Bestiary:

The fiercely aggressive jormungar are a marvel of evolution, possessing rows of chisel-like teeth capable of boring through solid rock, as well as muscular, flattened bodies well adapted to rapid underground navigation.

Legends tell of the nerubians' using the jormungar as creatures of labor, forcing them to carve massive tunnels through Northrend's subterranean ice and lay the groundwork for the nerubians' ancient civilization. With the fall of the nerubian empire, swarms of rogue jormungar ran amok, drilling through the region's permafrost, attacking surface dwellers, and devouring everything in their path.

Mythological Basis

The name is a reference to or inspired by the World Serpent of Norse mythology, Jörmungandr, which was said to be so large that its tail had to be stuffed into its mouth to prevent it growing larger than the world.

Speculation

This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials. It should not be taken as representing official lore.

It is speculated (given the carvings on the pillars around Uldum) that this wyrm, or one like it, escaped from the Uldum complex in Tanaris. The door to the complex seems to have a giant hole in it, sticking outward.

• There are several holes with this theory now that more information on the worms is available from the official website. Firstly, it is said that there are many jormungar and they have been around since the early days of the nerubians. Second, they are native to Northrend. Last but not least, it is stated they are able to effortlessly bore through solid rock and ice. It would make no sense to attempt to hold jorumungar in a stone prison. They would simply tunnel their way out.