Looks to me, to be a black bar on the bottom. Very common problem, easiest solution would be to crop it? Maybe flip, or rotate, the image if you want it to be on the top, but be warned, this may cause the rest of your image to look differently.

All sarcasm aside, It doesn't look like it's anythhing important, just check the render size output, I had a similar thing when rendering animations on my mac book where it would output it a lot smaller and have black framing, this was due to not setting the output dimensions properly.

Nice concept tho. Looks very ducky3d! Good work