It is known that exposure to carbon monoxide is harmful to animals because

Carbon dioxide is a harmful poisonous gas. It has more affinity towards haemoglobin than oxygen. When a person inhales Carbon Monoxide, the Carbon Monoxide binds to the Haemoglobin and forms a very stable combination called Carboxy-haemoglobin. It leads to suffocation as carbon monoxide does not disassociate from haemoglobin which makes it unable for oxygen to bind to the haemoglobin and hence reduces oxygen transport.