The tiger is a dangerous animal.
Because being dangerous is prototypical of all tigers. If something is called
tiger, it will be dangerous. The tiger means the species of tiger.
A whale is a mammal.
Because
being a mammal is one of the characteristics that define whales. For an animal to be called
whale, it must be a mammal.
The whale is a mammal. That's OK, because the species of whale belongs to the group called
mammals.
Tigers are dangerous animals. Whales are mammals.
Both are OK, because that's what people think about them and both are natural generalizations.