At one time, researchers thought it was a myth that polar bears could use stones to kill walruses. Not any more:

Walruses, weighing as much as 1,300 kilograms with huge tusks and nearly impenetrable skulls, are almost impossible for a hungry polar bear to kill. But new research suggests that some polar bears have invented a work-around — bashing walruses on the head with a block of stone or ice.

For more than 200 years, Inuit in Greenland and the eastern Canadian Arctic have told stories of polar bears (Ursus maritimus) using such tools to aid in killing walruses. Yet explorers, naturalists and writers often dismissed such accounts, relegating them to myth along with tales about shape-shifting bears.

Gloria Dickie, “Polar bears sometimes bludgeon walruses to death with stones or ice” at ScienceNews (July 29, 2021). The paper is closed access.

Here’s a video of a polar bear throwing a block of ice at a seal (the camera was attached to the bear; hence the bumpiness):

It’s unclear why some researchers believe that this behavior was folklore because the use of rocks or stones as weapons has long been noted among birds:

Traveling in Tanzania on a National Geographic Society photographic expedition, Zoologist Jane Goodall and her photographer husband Hugo van Lawick came upon an abandoned ostrich nest. Two ostrich eggs left in the nest were under attack by a variety of vultures, which were trying vainly to peck through the tough shells. While the Van Lawicks watched and photographed, they reported last week in Nature, two Egyptian vultures took a novel approach to their problem.

Picking up small stones in their beaks, they raised their heads high, then whipped the stones in the direction of the eggs with forceful movements of their heads and necks. Throwing, you might call it. After several hits — and many misses — they successfully broke open the shells and feasted.

Zoology:, “Birds that Throw Stones” at Time (January 6, 1967)

Here’s a seagull cracking open a marine shell with a stone:

From Phys.org we also learn,

Crows have been observed using sticks to hook prey, for example; otters use stones to crack open shellfish; elephants have been observed moving rocks and logs to cover watering holes.

Bob Yirka, “Pigs observed using tools for the first time” at Phys.org (October 8, 2019)

The use of rocks for killing prey can be seen as an inversion of the practice of dropping the prey in order to kill it or break it open. The intelligence needed is the ability to reverse a process.

In a discussion of the Lammergeier (Bearded Vulture of the Pyrenees) dropping bones from prey to crack open the marrow, we learn:

At least 23 other species including gulls, crows, and eagles take advantage of rocks and pavement to crack into nuts, mollusks, and other hard-shelled food. It’s a clever technique birds use to extract otherwise inaccessible calories, and one that involves more sophistication than might be apparent.

Priyanka Runwal, “Gravity Gives These Birds the Drop on Tough-to-Crack Foods” at Audubon (February 12, 2020)

Here’s the Lammergeier at work:

The Lammergeier learns the technique by trial and error. Orcas (killer whales) may similarly throw seals into the air. The seal, stunned by the impact on the surface of the water, is easier to despatch:

While we might assume that dropping the prey on a rock, for example, precedes bringing rocks to smash the prey, we don’t really know that for sure. Animals and birds don’t keep histories. They are more intelligent than we once thought — but not in quite that sense.

You may also wish to read:

Crows can be as smart as apes But they have quite different brains. The intelligence doesn’t seem to reside in the details of the mechanism.

and

Scientists clash over why octopuses are smart New findings show, the brainy seafood breaks all the rules about why some life forms are smart.