....I have found that Unicorns pretty much NEVER show up! Do they only show up in Spring or Summer when it's not cold? Where do they usually show up in Sunset Valley?

Also, a few expansions installed ago...I have noticed that Ghosts hardly ever show up in the graveyard as well! They used to pop out every night in the Sunset Valley graveyard and now it almost never happens. AND if I play another world and there is a graveyard, I NEVER see ghosts show up there, EVER!

Are there some changes that have been made to the game with the updates that I don't know about? Help! TIA

Oh and I have taken out ALL CC that can cause problems.

Unicorns show up on lots with water (lakes, ponds etc), at least I've heard something like that and in my game I think most of the time it's true.
They do show up but I use Twallan's Register:

http://nraas.wikispaces.com/Register

to help me with animal control etc. I hate horses being stuck and making my game slower.

I know they show up in places with water, but they used to show up every other night on my sims game and now it never happens, after installing Seasons........

I think that you are just unlucky at the moment. The last time I paid attention to this I found the unicorn on an almost daily basis. As this Sim had a maxed out wildlife friend skill and a pond on his lot the unicorn came to visit quite regularly.

I've noticed this as well. Ever since I installed Seasons, I see deer far less and I barely ever see unicorns now, even with placing the Swamper house near the waterfall. Wild horses happen nearly every day, as do cats and dogs, but I haven't seen a unicorn in several weeks. Perhaps it is just bad luck or selective perception?

they ONLY show up in places with water.