There are only 2 cheats (by renaming exhibits) to get extra animals: Xanadu (for the unicorn) and Cretaceous Corral (for the Triceratops). You can get mermaids by dropping mermaid statues in tanks, but beyond that, I don't think there are any more "secret" animals in ZT1.
For downloadable dragons, I recommend Aqua Dragon's "European Extravaganza", which includes the Emerald Dragon, and the Red Dragon by Lehner (my 2 favorites). Both are found right here on ZA