The writers said that it's called the "Tatzelwurm".
As you can obviously tell, it's a giant worm. To me, it has characteristics of the Shai-Hulud from "Dune" and the Graboids from "Tremors". (Shai-Hulud: three-way jaw, Graboids: mouth tentacles)
(Oh, and by the way...the reason why Discord got really sick when the Tatzelwurm sneezed on him is because, according to legend, the Tatzelwurm's breath contains enough diseases to kill a human.(Discord, being a reality-warping dracoequinus, is obviously MUCH more resilient than a human.)