One of the interesting stories I found recently which was used as an example of why extinction is such a bad thing for us, was about these kinds of chimeric medicines.
One of the best animals for antibody and hormone engineering is the pig, because they have a very similar body structure to us and are so common. But, when they started using pigs to create insulin cells for humans, they realized that almost all modern pigs have a disease that can be transferred to humans.
Then, it was discovered that there was a type of pig on an island group that was being cleared for native birds. This pig species had been isolated from other pigs for generation and didn't have the disease. They almost made it extinct, because they were trying to clear the island, but luckily a few were saved and now they are the only way to create this chimeric medicine that could save thousands of lives.