In The Most Dangerous Game, what does General Zaroff let out at night to ensure no one escapes the island?

General Zaroff hunts human beings for sport.  He finds them challenging because they are the only animal that can reason.  When Rainsford expresses shock at the idea of the game, Zaroff explains that the person is pursued for three days.  If he can survive during that time, he wins the game.  So far, Zaroff has not lost one game.  Zaroff did admit,

“Occasionally I strike a tartar.  One almost did win.  I eventually had to...

General Zaroff hunts human beings for sport.  He finds them challenging because they are the only animal that can reason.  When Rainsford expresses shock at the idea of the game, Zaroff explains that the person is pursued for three days.  If he can survive during that time, he wins the game.  So far, Zaroff has not lost one game.  Zaroff did admit,

“Occasionally I strike a tartar.  One almost did win.  I eventually had to use the dogs.” (pg 6)

At that time, Zaroff escorts Rainsford to the window.  There he sees about a dozen or so huge black shapes moving around in the courtyard. Zaroff explains that

 “…. They are let out at seven every night.  If anyone should try to get into my house --- or out of it--- something extremely regrettable would occur to him.” (page 6)

This is a warning to Rainsford.   He will not be allowed to leave during the night because the dogs will attack him and rip him to shreds.  Zaroff does not utilize the dogs unless the quarry he is hunting is winning the game. Rainsford will have to deal with the dogs later.