The Ursa. A creature explicitly designed to be a killing machine against humans. Its design plays on audiences' primal fears; it's insect-like, it's huge, you can't predict when it senses you, and it kills any living thing it "sees" indiscriminately. The way it quickly evokes the fear pheromone in its prey? Post dead humans, hanging and impaled, in plain sight.
The slug-like creature that bites Kitai, and the gruesome effects of its toxin. It's even more horrifying to those who are aware of somewhat similar creatures already living in today's world. We can literally see the effects of the poison at work, including his face visibly swelling from the venom.
Kitai's dream on the raft, where he talks to his loving Ranger sister starts out sweet and endearing, but then slowly builds tension and suspense until she reveals the urgency of his situation through her shocking near-mutilated corpse.
