Sims 4: I have two mailboxes and a unicorn that I can't remove

I had the same kind of issue with a rug, and fixed it by accident! (lol)

I was saving individual rooms from my house to recreate the house on a different lot, hoping to fix it, when I went to grab a room the rug had been covering, I ended up grabbing it and deleted it from there. Maybe if you save a couple rooms you might be able to grab it in build mode. Good luck