This is a great question. When reading a text, it is always important to look at the context.  Therefore, when we look at the context of these words, general Zaroff is explaining to Rainsford that he developed a new animal to hunt. At first Rainsford is perplexed—a new animal? 

As the conversation continues, Rainsford understands that Zaroff is hunting humans—according to Zaroff, animals that can reason. 

Here is the dialogue:

"I wanted the ideal animal to hunt," explained the general. "So I said, `What are the attributes of an ideal quarry?' And the answer was, of course, `It must have courage, cunning, and, above all, it must be able to reason.:

"But no animal can reason," objected Rainsford.

"My dear fellow," said the general, "there is one that can."

"But you can't mean--" gasped Rainsford.

"And why not?:

"I can't believe you are serious, General Zaroff. This is a grisly joke."

After this, Zaroff goes into details about how all of this hunting takes place.  For example, he gives the huntee a good meal, a head start, and some supplies.  Rainsford is shocked, but unfortunately Zaroff is dead serious.  It is within this context that Zaroff wants to show Rainsford his collection of heads.  In light of this, the collection of heads is human.