What was the “Sea Horse”? What “trial” did it face, and what was the outcome?

The Sea-horse is a walrus. It faced the trial of being hunted...... a trial in which it failed.

Then I came on a herd of walruses, and I did so long to make a prize of one that I forgot everything else, and made chase after them when beyond the influence of the current, until I got near enough to harpoon this fine fellow. He swam more slowly, and I struck him a second time; then he sought refuge among these rocks, and expired. I landed, and scrambled to where he lay; but I took care to give him the contents of my pistol before going close up, having a salutary recollection of the big serpent's parting fling at you, Jack.'