Cheat Set Up
Select an exhibit. Choose the oldest exhibit that you can as using a brand new exhibit can cause the game to crash. Delete the name of the exhibit and replace with a cheat code.
Cheat Code
Rename the exhibit either "mystery island" or "fire reptile". Wait for a few minutes to see that the game is still running as normal. Save the game.
Accessing the Dragon
Go to "Adopt an Animal" in the "Gameplay Menu". The dragon will be available to adopt just like the other animals. Select the dragon and place it in one of your exhibits.
Influence
The dragon, or dragons are very popular with visitors and will bring you in plenty of prestige and extra cash. Why not ramp this up even further with some more mystical animals? Use the same technique as that which you used to unlock the dragon but name an exhibit "Xanadu" to get hold of a unicorn.