I've written a number of essays about grief and mourning in nonhuman animals (animals) and just today I learned of a most heartwarming video of a dog named Bella deeply grieving the loss of Beavis, her beaver friend.
Here's a brief description of Bella and Beavis's close friendship.
"Before Beavis passed away, he and Bella were inseparable. They ate together, played together, and even shared living quarters. Beavis passed away in 2012, but the pair's story resurfaced after a video that the owner shot of the two appeared on Reddit.
"In the heartbreaking video, Bella lies by the side of her deceased companion and appears to cling to the idea that Beavis might just sleeping. As Bella seems to realize that her friend is not coming to life, she whimpers, nuzzles, and licks her friend as if trying to say goodbye."