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

i did found a easy way out  

if you  try the mailbox head on your sims. here  the trick wile he has it

still in testingcheats on ,  click wiht shift  click on the sims then select reset the mail box will drop on the floor 

then go in edit  house and  just click on it as if you would move it but in the bottom of the screen there is  sell option that gives you 0$ just sell it i did it on my sims 4  and it worked for your unicorn maybe try the reset  option or  the sell option the same as this one .