As humanity has grown they have pushed uncountable species to extinction and before that, the planet itself pushed billions to extinction. But now, with astounding new technology, we can combine the ever-present humans with the DNA of extinct and endangered creatures, transforming them into sexy anthropomorphic creatures that look and act like the original versions to none transformed humans.
Now there just lays one more problem in before you can change the unknowing masses of the zoo into the long-forgotten creatures. Who to transform first? Perhaps the family over there would make a good Direwolf pack. Perhaps those college kids who are goading the horses would make good mammoths.