Eleven IS the monster. Everything in the Upside Down is a dark and scary reflection of what is above. She's a child, she fears monsters. She fears herself because she knows she's not normal, she fears that dark place they make her go to, and that manifests as her Upside Down version (she is a powerful psychic after all).
Look at the final scene, she holds her hand up to it, it holds it hand up to her in the same way, and she cant exist without it (hence they both are destroyed). Also she even says multiple times "i am the monster".
I think there's some weight to that theory. But I think a lot of the evidence eg. 'I am the monster' is not to be taken literally. I always saw the upside down as some alternate reality where the cold war did in fact culminate in nuclear strikes causing a global nuclear winter of sorts. Can't really explain the monster in that same context though. I think there's quite a few plot holes/unexplained events and phenomenon there going to HAVE to explain in series 2.