The nearest zoo to us has a Halloween event every year but it's always right before Halloween not on the actual day. My niece Little K got to go this year to have fun. I think she was dressed up as Anna from Frozen, she had a jacket on so it was hard to tell.
There was plenty of Disney stuff.
And Princess Peach with the worst drawn-on face I've seen in quite some time.
And there were "real" princesses there too.