Were you hoping that there were a unicorn spawner similar to seed/fish etc. spawners? Unfortunately there is not any spawners for unicorns. The game generates them just like it would generate townies.
With this mod you can get higher change of unicorns spawing at those fishing spots
http://www.modthesims.info/download.php?t=459352
With Nraas master controller mod you can turn your own horse into unicorn, and I am sure there is somewhere a command to summon one, just cant remember how at the moment.