Eep's opening narrative on how all the other cave families in the area died out. Even before we see a single CGI animal, the 2D animations of the neighbors' ugly fates - particularly the "mosquito bite" incident - establish that there are nasty things out there in the Croods' world.
At the beginning, Grug uses Sandy's teddy bear as a prop to tell a story that deters any sense of curiosity or whimsy in his children. How? That bear was alive until the Croods killed it and repurposed what looks like it mummified corpse as a family toy.
"The End". There's no explanation as to what's causing it or why (though the opening implies its due to continental drift), but Guy was not exaggerating when he called it the End of the World.
When the first earthquake hits, Grug yells for his family to get to their cave - but as they approach, a massive sheet of rock falls from the cliff above. Grug, the only one who sees it, frantically screams for his family to stop, but they don't hear him.
The Piranhakeets. Swarming birds that can skeletonize an animal in just a couple seconds. Arguably the most dangerous animal in the entire movie.
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 remembers that it's terrified of the dark, and immediately helps Grug revive the embers so they can cling together in the comforting torchlight.
