I could be wrong here but my understanding is if a company is acquired they WERE a unicorn (if the acquisition was for $1B+) but they aren't a unicorn after the acquisition.
I go the other way. Your valuation means jack until you are acquired . It's just some VCs thinking you've built $1 billion business. For example YikYak being valued at $400M. Actually getting acquired for $1 billion... now that's special and rare