Thanks that's great news.
The plan will come true then. Let me try to explain it a bit better ...
You have a sim (or two) and one horse with the correct coloring and design but you want to breed unicorns. So you befriend one and get it into your family (hopefully with another gender as your horse). They get a foal and if it has the correct coloring and design like the horse but is a unicorn you have a half blood. So you release the adult unicorn after BFF of course and sell the adult horse. Then catch another unicorn and breed them together again. The next foal will be a 3/4 blooded unicorn. The next would be a pureblood ... at least in theory. The last is the unicorn you want with the right colors and the correct design.
Uh ... that sounded even strange to my ears.
Of course you could easily just breed two unicorns together. That would be much easier.
I'm sorry my mind is a confusing one ... even for me.
Oh and just one more question. How will you keep them in your neighborhood? Is it possible for another family to visit, befriend and finally adopt the unicorn someone else has on their farm? Would that be possible for a unicorn foal as well?