Engorged Blight Worm

The Engorged Blight Worm was a monstrosity created by Putress and his fellow Apothecaries in an attempt to stop King Varian from reaching the Apothecarium, Putress' Headquarters.

Eventually, the worm is destroyed by Varian, Jaina, adventurers, and Varian's warriors during the Battle for the Undercity.

Speculation
Despite the dissonance in appearance, it may be the slime-producing construct normally next to Ganoosh, due to the construct's conspicuous absence during the battle and their similar body shape.