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.
Nightmare Fuel: "The End". There's no explanation as to what's causing it or why (though the openning implies its due to continental drift), but Guy was not exaggerating when he called it the End of the World.
A case that is initially played straight: Grug after going into a cave and drawing a cave drawing of his family, only to have the saber tooth from earlier show up and try to eat him, and then blows out the torch to presumably eat him off screen. Which is later defied when the cat cuddles him.
The Piranhakeets. Swarming birds that can skeletonize an animal in just a couple seconds. Arguably the most dangerous animal in the entire movie.