One hungry polar bear from the 6th episode attempted to attack a group of walrus. Its desperate to get a kill in summer. Unfortunately the walrus herd is just to powerful and basically impenetrable. Yet the polar bear keeps trying before finally succumbing to its wound and dying.
From the same episode, the flock of penguin chicks freezing to death in the storm.
The young elephant that loses its way in the first episode.
Arguably the series as a whole. As climate change become more and more affecting the planet, most of these creatures are facing greater threat of extinction. A fact which David Attenborough constantly reminds us through out the series.
Planet Earth II
The turtles in the "Cities" episode. Unlike the other wildlife to feature in the episode, which have successfully adapted to city life and flourished more that their country counterparts, the turtles find themselves confused between the city lights and the full moon reflected on the water and end up going into the city, straight into the paths of predators and dangers that kill them.
Luckily, this was one of the very rare times the producers decided to interfere with the wildlife. Once they got the shots they needed of the turtles stuck in the drains or halfway across a busy street, they scooped them up and returned them to the ocean. Usually they never step in when an animal is in danger due to the need to depict a genuine encounter, but in this case they pointed out that the problem was man-made so it was entirely justified for humans to interfere to undo the damage.
In the "Islands" episode, a small bird begins chipping away at an unattended fairy tern egg and breaks it open to feed. When the mother returns, she clearly recognizes that there's a problem, but her instinct to incubate the egg is too strong. The image of the mother sitting on top of her broken egg, yolk dripping down the branch, is absolutely heartbreaking.