The name of the new animal that General Zaroff hunts is Homo sapien. In layman's terms, General Zaroff has started hunting humans.
While talking to Rainsford, General Zaroff admits that he is bored with hunting all other types of prey. He is no longer excited by hunting things like lions, tigers, and bears (oh my).
"They were no match at all for a hunter with his wits about him, and a high-powered rifle. I was bitterly disappointed. I was lying in my tent with a splitting headache one night when a terrible thought pushed its way into my mind. Hunting was beginning to bore me!"
Basically, Zaroff feels that hunting has become too easy to be exciting. The problem, as Zaroff sees it, is that the animals operate on instinct. He believes that instinct is no match for reason, so Zaroff needs to find and hunt an animal that can reason.
"It must have courage, cunning, and, above all, it must be able to reason."
Rainsford quickly responds by saying that no animal can reason. It's then that Zaroff tells Rainsford that he now hunts humans because humans can think and reason.
"That is why I use them. It gives me pleasure. They can reason, after a fashion. So they are dangerous."
In fact, Zaroff stocks his island with human prey. He deviously captures ships in order to capture crew members. Zaroff then feeds the prisoners and exercises them. That way his prey are in peak physical condition before being hunted.