Once you reached Friends it’s safe to click Friendly, Ask to Join Household. The Unicorn will ask your Sim to prove him/herself Friend of All Creatures. For this you will need to become Best Friends Forever with 3 Household pets of any kind. A combination of Horse, Dog, and Cat, 3 Horses, 2 Dogs and 1 Cat etc. After your Sim completes this task, sadly, you’ll have to wait till your Unicorn Friend appears again, so keep an eye out! Once you find your Unicorn ask it again to Join your Household and it will accept. Congratulations! You now have a Unicorn as part of your household!
Something to consider for the guide:
What happens if you've already BFF with 3 pets the
first time you ask it to join your household? Will it join then, do you have to ask a second time, or do you still have to wait until the next day?
All this unicorn talk has got me wanting to go find one now.
(Darn it, I'm not a follower of the crowd! I'm a
trendsetter! Look what you've done to me...
{lol})