This poem is on the weak the side, and there are quite a few quirks, but some phrases you have here are written well enough. I like the use of a more sophisticated language, or at least what you tried to use, specifically " 'Tis not..." but the rest of that line ruins the first part of it, simply because the rest is more awkward and in the context of the entire poem sounds forced. I can't help but think you could have substituted a grander ending to that line but in such a way as to not ruin the rhyme of the next line "But an aperture to the things that hide.", which I think is superb.

I also think you're entire second line is unecessary altogether. Its obscure in placement and in substance, because you start off the poem asking about the monster, then interrupt that thought by declaring that you are the monster, but then break off again describing it as if it were some unidentified being needing description until the dramatic last line that is supposed to leave the greatest impact, but can't do so because you already took away the surprise. If the second line were not there, I think this poem would be that much better, because if you were to read it through without it, the effect on the reader is that much more significant.

Sorry that last bit was a mouthful. Anyway, good poem, be proud.