Valkrionn
The Hamster King
Because 1) that is a fun event.
2) Kael (or someone else from The Team) made a text showing that at least 1 unicorn existed once on Erebus... for 2hours. So the unicorn existing is not against the princess rule. The princess rule only says it has to have a bad ending .
Your point #2 is false. It was written by KillerClowns, who is not on the FfH team. He is on the RifE team.
Honestly, I'd be okay with Unicorns... If they were essentially nightmares, with a giant, blooddrenched horn and blood dripping from their canines.