These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: The literal spiders (aka Zigmos) can make some of the later levels unwinnable if you lose your weapon. There's also those pesky frogs/Kellos. The literal Goddamned Bats can be particularly demonic when platform jumping. And the jumping swordfish come at you almost-nonstop in the later ocean levels.