The theory of the survival of the fittest suggests that species adapt to the environment to survive. For example, insects may develop colorful patterns on their wings to scare off potential predators. The biological foundation for such changes and adaptations can be gene mutation.
Then a question arises: suppose mutation can trigger some effects so that an animal suddenly becomes poisonous. Why the poisonous feature cannot stay like the colorful pattern on insects? Fortunately not, if yes, then many things we see today will be poisonous!
One explanation is that the colorful patterns are salient features, so they can immediately be effective to protect the insects. In the long run, this feature can be passed onto future generations. The poisonous feature is not salient. So a wolf will continue to eat that poisonous rabbit and there is no chance for the poor rabbit to pass on this great feature.
In a sense, these poisonous rabbits are like experience goods. Before you consume them, there is no way for you to know their quality. You only learn about their quality after you eat them, but then it is often too late for you to regret. It is a blessing for us that there is no review systems (e.g., those similar to Yahoo movies, Yelp, etc.) in nature. If yes, then the poisonous rabbits can pass on the feature and today we will have to face many more poisonous animals.