Hullin, Chapter Three, Mishnah Five
This mishnah provides a few more criteria for determining if an animal is a terefah.
1) [If an animal] suffered from congestion of the blood, or was overcome by smoke or by a cold, or if it ate oleander or chicken dung, or if it drank noxious water, it is permitted.
2) If it ate poison or was bitten by a snake, it is not forbidden as trefah but it is forbidden as a danger to life.
Section one: Although an animal who suffers from one of these problems, or ate or drank one of the things listed here may be in some danger, it is not enough in order to consider the animal a terefah. While these might cause it to die, they might not. Oleander is dangerous to an animal.
Section two: Commentators understand the poison here to be something that is poisonous to people but not animals. If it was poisonous to animals, it would certainly render the animal a terefah. The snake bite also must also be understood as a bite not strong enough to kill the animal. Since these things will not kill the animal, the animal is not a terfah.
Although the animal is not a terefah, its meat is forbidden because it is a danger to human beings. The rabbis took very seriously the prohibition of causing danger to oneself. Eating meat that might cause death or other harm is not just foolish, it is religiously forbidden.