This is the animal upon which the woman was riding (Revelation 17:3). The reference to the “seven heads and ten horns” (Revelation 17:3, Revelation 17:7) makes clear that this is the same beast as John had seen in Revelation 13:1—the dragon’s first colleague in his war on the saints (Revelation 12:17; Revelation 13:7). That the woman is riding this beast is to say that she and the beast are united in their purpose. She may be its mistress, but the beast has its own mandate and agenda. In the commentary on Revelation 13:1, it was concluded that this beast represents the human heart as its sinfulness receives expression through force. That the woman rides the sinful human heart to lure people to commit spiritual adultery in relation to the divine Spouse is no surprise.
8 The beast that you saw was, and is not, and is about to rise from the bottomless pit and go to destruction. And the dwellers on earth whose names have not been written in the book of life from the foundation of the world will marvel to see the beast, because it was and is not and is to come.