I've adopted a unicorn easily with my sim Cheri Montalvo, who was attempting the Ark Builder LTW, since she owned two cats, a dog, two horses, two birds, and some terrarium pets. My Sim had interacted with so many other species of pets, that the unicorn readily accepted her offer to join the household.
My other Sim Buckshot Thibideaux was going for The Jockey, and had befriended ten horses (to complete the Riding Challenge), a couple of strays, and carried around one crow that he interacted with daily. No problem befriending a unicorn.
Another Sim Billy Roy was friends with a lot of horses he'd raised and sold, and also regularly interacted with any deer, raccoons, and large birds that came into his yard. Some of his horses had the Friend of the Herd LTR, and a Unicorn came right into his yard! He befriended her the same night. And that was in Bridgeport!
There's an app on the App Store now for iPhone users all about Horses and Unicorns, called Equestrian Center - Horse Guide for The Sims 3: Pets.