Nice scares! I found the monster very effective, but very fair. It made me scramble a bit whenever I saw it, but it never felt like an instant-lose scenario where I couldn't do anything. I also found that it showed up just enough to build tension without becoming silly. Very well executed on the horror front.
One critique I have is that I found that there were maybe one too many chests. At the end I found myself looping around the same spots trying to find the one last chest, and the tension was bordering on becoming annoyance, but I don't think it ever actually reached that point.
I'm gonna echo the comment that said to avoid installers. A game this small doesn't need to create star menu items, desktop shortcuts or a user-space data folder. It's an extra step that may cause less people to play your game than otherwise would.
Great job overall!