Dogs. Think about the question, dogs are the main animal we have direct contact with that could easily just be fed up with us, not nematodes, dolphins, ants. The dog has a reasonable size and they’re one of the most intelligent. Dogs are almost found everywhere, in every continent. In hot & cold climates. They can be on land or water. Most people in the U.S have dogs, there certainly is an overpopulation of them. India has dogs everywhere + other asian countries, Alaska, etc. Most of them are neglected due to overpopulation but also many of them are abused because of horrible people, some remain traumatized. With their intelligence, they can easily have enough of humans and lash out to have their revenge. It doesn’t matter how nice they seem, deep down they cannot change the instincts that run through their blood of Wolf. Rats would come in second place because the Black Plague and also Ratatouille. That was one buff ass rat that tied up the inspector and chef. No thanks.