Zoo Tycoon: Complete Collection Easter Egg - Extinction of the Dinos

First to get to this egg you must click on "Get New Items" (right below "Continue Saved Game") on the main screen.

Then click on "Check Internet" (make sure you're connected), and download the reindeer (it might take a while).

Once you have it downloaded open a game that already has dinosaurs in it, and puchase some reindeer.

Name one of the reindeer "Comet" and all of the dinosaurs in your zoo will disappear!