So you really really want unicorn, but it's so hard to befriend the animal(s). No problem: there's an easy shortcut.
Step 1: Press Ctrl + Shift + C and type in "testingcheatsenabled on"
Step 2: Find the unicorn (hint: look for the aurora)
Step 3: Press Shift and click on the unicorn. Select "Add to active household".
You are done.
Warning: trying to get unicorns can be seriously addicting.
Note: you can only have six unicorns per household.
Need help locating unicorns, look here:
http://www.carls-sims-3-guide.com/pets/unicorns.php