Everything. From natural disasters, hunting, predation (eat or be eaten), diseases, accidents, encounters with other animals, etc…even without human intervention or if we didn't exist.
A Rocky Mountain goat falling off a cliff.
A baby monkey falls from a tree.
A beaver crushed by the tree it felled.
Animals washed ashore during storms… and whales and other animals disoriented and stranded.
A bird in a spider web.
Chytridiomycosis, or amphibian fungus, has caused a massive die-off of these animals around the world.
Extreme weather such as cold.
Or heat, like the heat wave in Australia that killed hundreds of flying foxes.
Epidemics like the bacterial one that killed hundreds of thousands of saiga antelopes in the steppes of Kazakhstan a few years ago…
To give two more examples, in 1955 the passage of Hurricane Janet in the Atlantic (one of the strongest hurricanes on record) caused costly material losses and took the lives of hundreds of people.
The hurricane's impact on the small Swan Islands off the northeast coast of Honduras destroyed the only habitat of the Swan Island hutia (Geocapromys thoracatus), a rodent related to the guinea pig. This, along with the subsequent introduction of domestic cats to the islands, led to the disappearance of this animal, which was unique to the archipelago.
It is believed that the eruption of Ecuador's Tungurahua volcano in 2010 wiped out the only known population of the Lucihormetica luckae cockroach, which was only found in the vicinity of this location, and all trace of it has been lost ever since.