With CC you have a total of 4 mystrey beasts
#1 - Unicorn (Unlocked in Paradise Island Scenario also by re-naming exhibit "Xanadu")
#2 - Triceratops (Unlocked in Breeding Pandas Scenario or renaming exhibit "Cretaceous Corral")
#3 - Deinochus (Unlocked by Completing all DD scenarios or Renaming exhibit "Super Croc")
#4 - Mermaid (Unlocked by placing a mermaid statue in a tank)
I am 99.9% sure there are no more mystey beasts. There are possibilities, ZT1 is'nt that old of a game, It's only about 4 years old. And after the big "Mars Map Discovery" I saw at a site there could be a mystrey beast #5.
I have no links but someone has a screenshot of a alien-type creature found on their zoo tycoon map "Mars"