She wants it

Instinctively, the sight of that massive cock, 15 or 16 inches long, turns her on. She whinnies invitingly, and flicks her tail back and forth, exposing her vulva. The unicorn stallion neighs at the sight. He approaches her at a slow trot, and nuzzles her side.

The thought of fucking this gorgeous animal, with its even more amazing cock is making her sopping wet. Somehow, she instinctively knows what to do. Her tail moves to one side, and she shifts around so her pussy is right in the stallion's face. He takes on sniff and neighs loudly. The snow white unicorn rears up and mounts her, the first few inches of his long stallion cock sliding insider her dripping mare pussy. Then with a single explosion of movement, the unicorn stallion buried all 16 inches of his cock inside her. Kelly is shocked by how far it is inside her, and how roughly the stallion is using her pussy.

Within a few minutes both Kelly and the stallion are cumming, she can feel what feels like a bucketful of sperm flash inside her pussy as the walls clamp down on the unicorn's cock while she cums.