Hi,
Hmmm, that didn't work for me...
From the moment the "Sensational Sparkles" goal came along I bought a Gem Cove and had it empty for a loooooooong time in my forest; until some 2 months ago I decided to sell it as I needed the space. I too secretly hoped that that would trigger a Unicorn for me, but alas, no luck. Then recently when the grand habitats were on sale, I tried the reverse logic again and boldly bought a Grand Gem Cove, hoping that that would help (it did for Sandybellee who did the same!) but so far it hasn't worked. I did finally breed my first ultra rare though: a Golden Retriever!
Enjoy your unicorn!
Olafo