What does Rainsford see in the bush once he begins walking on the island

Rainsford stumbles upon a scene where something large, probably an animal to his logic, had been injured and fell into the jungle underbrush writhing around and suffering. He also found an empty gun cartridge which indicated this animal's injuries were from being shot.

Explanation:

This quote from the short story serves as evidence for the answer.

'Some wounded thing--by the evidence, a large animal--had thrashed about in the underbrush; the jungle weeds were crushed down and the moss was lacerated; one patch of weeds was stained crimson. A small, glittering object not far away caught Rainsford's eye and he picked it up. It was an empty cartridge.'