Why was the unicorn invited?

Answer 1

The unicorn was not a species that was invented. Though people don't have much proof, I think there were possibly unicorns before. It was rumored that they used to have virgins sit alone in forests or somewhere far away from "civilization", so that unicorns would approach them.

Even if there are no such thing as unicorns, there must be something, somewhere, in the billions of universes and galaxies, that resembles a unicorn.

The unicorn represents purity, eternal life, wisdom, health and knowledge.