I am not very sure if it is really a hypertext-system.
In the following I refer to the only Animal-Game I know of. Maybe there is another thing called AnimalGame.
Animal is a learning program. It learns from the people who play the game.
Here you can find the full story: http://www.fourmilab.ch/documents/univac/animal.html
Here you can play it: http://www.animalgame.com/ (If you want to teach it, you will have to sign in.)
http://www.20q.net/ is a very impressive system built on similar lines, which plays the "animal vegetable or mineral" game, very effectively. (Wow, thank you for this hint, this is amazing! And fun! Beats any human beings!)