Article very much based upon http://www.allyngibson.net/kzin.html . All good bits were written by Allyn Gibson and anything that looks a bit naff should be blamed on me. MikeLacey
The Animated Series was produced in 1973 and 1974 and many fans discount it. The animation, it has to be said, was a little hmm... It's been said that "those fans who came to Star Trek in the 1980s when Star Trek: The Next Generation began and those that came after are the ones most likely to reject the Animated Series as a part of Star Trek canon." 
One thing that sets the Animated Series apart for Larry Niven fans is the episode The Slaver Weapon, adapted by Niven from a story called The Soft Weapon (published in Neutron Star (collection)) at Gene Rodenberry's suggestion.
The Animated Series took advantage of animation's ability to have non-human like aliens with the Kzin and Niven inserted the Kzin into the Star Trek universe in the Animated Series. With that, aggressive cat-like aliens stepped forward to challenge the Federation and its Starfleet and various games and films have been lightly peppered with Kzinti references ever since.