sims 3 unicorn

@Nena16lovex  Yes, I can see it fine.  The most obvious issue is that you have zero free space on your C drive.  I'm guessing data is being redirected to D (it looks like this is one drive partitioned into two), or else your computer would have stopped working a long time ago.  Still, you need to free up at least 20 GB on C before you can rule that out as an issue.

However, your computer's hardware isn't really strong enough to handle Pets, or any of the more demanding packs.  That's not to say you can't or shouldn't play, and this system has survived a long time.  But it's possibly that no unicorns is just a symptom of the hardware not being able to keep up with the demands the game places on it.

You could test by starting a new save in a clean folder.  Move your existing Sims 3 folder out of Documents\Electronic Arts (and definitely onto your D drive—if you need to, just create a folder on it called Backup or something), then launch the game to create a new folder.  Start a save in Appaloosa Plains, the world that came with Pets, and see if you can spot a unicorn.

Another possibility, although I'm not sure how likely it is, is that the unicorn is spawning but your graphics card isn't rendering the aura that you're looking for in Map View.  The card is quite old, and its driver is old as well, so it's possible that it's not doing its job here.  This is harder to test, unless you want to manually check every park, fishing spot, etc. in town every night.  There is a mod that can help search for sims on a map, although I've never used it specifically to look for unicorns.  Let me know if you're interested.