Imposter!

In Unicornia horses do exist and they are very jealous of the unicorns; to the point that sometimes they try to dress up as unicorns and play with them.
The unicorns don't stand for this.

Because unicorns are every color of the rainbow (and then some) some real unicorns are horse brown.
Their friends often make jokes about how they might be a horse. Just jokes, but it gets to them after a while.

Don't worry this one is clearly a horse.