Hello!
First, A unicorn car is an exclusive car, only gifted out by members of T10 to people under certain conditions. Unicorn cars are, like their real counterparts, extremely rare to find. So, Like in real life, FM4's unicorn cars are hard to find. (but, easier to get than in FM2/3)
This forum post should give you everything you need to know about Them. an excellent post by an excellent member of the FM community
http://forums.forzamotorsport.net/forums/thread/4958253.aspx
There are numerous ways to get them, none of which involve you to be a specific level. These include;
- Being a helpful member of the FM community
- Participating in the Rivals event where it states 100 of the Unicorn cars (the BMW M3 street version) will be randomly given out
- Community bounty hunter events which have a T10 member as the rival (keep looking on the General discussion page here to find out when they happen)
- Posting on the News section Johniwanna writes, when he is gifting them out (he is known as the Unicorn car saint in these places:P)
- WHen they return, being the celebrity guest in the King of the Track events
- or being a VIP member, where you periodically get gifted them
Hope this has helped, and good luck in hunting the 17 Unicorns!

FM4 & Horizon Unicorns - TOO MANY!
I give Cookies! (>'.')>#