I was searching for rocks, meteors, gems, seeds and what-not around the map while my sim was asleep and I came across a sparkly colourful light by one of the community lakes in Appaloosa and I zoomed in there was a horse who when I put my cursor over it the name 'Belle Horse' showed up. There was a glittering rainbow coming off of the horse and going over the lake, all of a sudden there was magical music and the horse disappeared into this colourful light!!! I'm assuming that is the rarest horse they can see, but I wondered if anyone else has seen it? Did you get your sim to interact with it? Is it magical? Can you adopt it?!

I want answers!!! Google doesn't have any

That horse you came across is a unicorn
http://forum.thesims3.com/jforum/posts/list/500309.page

haha thanks i figured it out shortly after i posted this! feel a bit silly now lol x