THE NIX

Samuel's mother told him about the Nix.  Another of her father's ghosts.

[...]

The Nix would appear to the child as a large white horse.  Unsaddle, but friendly and tame.  It bowed down as low as a horse was able, so the kid could leap onto it.

At first the children were afraid, but, ultimately, how could they refuse? Their very own horse!  They jumped on and when it stood up again they were eight feet off the ground and they were delighted - nothing this big had ever minded them before.  They became bold.  They would kick at the horse to go faster, and so it broke into a light trot, and the more the kids loved it, the faster the horse would go.

Then they wanted other people to see them.
They wanted their friends to stare with envy at this brand-new horse.  Their horse.

It always went liked this.  The kids who were victims of the Nix always felt, at first, fear.  Then luck. Then possession. Then pride.  Then terror.  They'd kick at the horse to go faster until it was in a full gallop, the kids hanging on to its neck.  It was the best thing that had ever happened to them.  They'd never felt so important, so full of pleasure.  And only at this point - at the pinnacle of speed and joy, when they felt most in control of the horse, when they felt the most ownership of it, when they most wanted to be celebrated for it and thus felt the most vanity and arrogance and pride - would the horse veer off the road that led to town and gallop toward the cliffs overlooking the sea.  It ran full bore toward that great drop into the violent churning water below.

[...]

When he (Samuel's grandpa Frank) told Faye (Samuel's mother) about the Nix, he said the moral was: Don't trust things that are too good to be true.  But then she grew up and came to a new conclusion, which she told Samuel in the month before leaving the family.  She told him the same story but added her own moral: "The things you love the most will one day hurt you the worst."

Samuel didn't understand.

"The Nix doesn't appear as a horse anymore," she said.  "The Nix used to appear as a horse," she said, "but that was in the old days."
"What does it look like now?"
"It's different for everyone.  But it usually appears as a person.  Usually it's someone you think you love."

The Nix, Nathan Hill