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.)
... That is Dune, right? I've never actually read it.
First the manticore, then Dragon, then Phoenix, now this...
Fluttershy: Equestria's greatest summoner!
Though seriously, I can actually picture her getting along with that thing
Alastor Soul: Summon ALASTOR the Familiar costs 5 MP to use, but drains magic
You figure that out all by yourself?