Wildlife documentaries can sometimes show you the more unsettling side of nature, but few have been so unnerving as the segment focusing on ants infected by the parasitic Cordyceps fungus. The intimate macro view of these creatures' distress as the fungus spreading through their brains modifies their behavior to suit its needs makes it seem far more personal than simply a bunch of minute insects, and that's even before the Body Horror kicks in as the fungus sprouts from the deceased ant's head, growing until it can release its spores across the colony below. And then, because that's simply not horrifying enough, they go ahead and show a montage of various other insect species' Cordyceps-sprouting corpses whilst the eerily haunting background music plays, the camera rotating slowly around each one to ensure that no detail is missed. YIKES. David Attenborough's matter-of-fact narration over the whole thing does nothing to mitigate its effect.
The episode which showed the exploration of underwater caves, specifically the moment when the diver tried to squeeze through the narrow gap. Claustrophobia mixed with the fear of damaging the life-support equipment to the max.
From "Forests", the squirrel screeching when the pine marten kills it.
Do not watch the "Caves" episode if dirt, bugs, or small spaces make you squeamish.
The chimpanzee battle shown in one episode is terrifying due to the editing and cinematography suddenly taking a number of cues from horror films. During the battle the low-angled, slightly shaky shots give you the distinct impression the film crew is actually in mortal danger. Once the battle is actually over, the shots are artfully framed so that you know chimpanzee cannibalism is happening without actually seeing much blood or flesh.
Planet Earth II
The "Islands" episode has the infamous snake chase scene, where a juvenile marine iguana stays desperately still in an attempt to avoid detection from a group of racer snakes. One of them gets close enough to detect it, and as the iguana takes off running, dozens of snakes pour out of the rocks like something out of a horror film. Enough to give anyone ophidiophobia. Even the crew were disturbed by the Zerg Rush.
From the sixth episode, "Cities", we have city leopards. The idea of a big predator stalking in your city is quite horrifying. The cameraman who shot that night-vision footage was out there alone when he filmed it. One very large male actually walked right up to the guy's observation-blind.