This looks like the type of horse that will lure you onto his back and then carry you into a lake.
Kelpie. Or possibly a Pooka. Do not trust horses who show up in the middle of nowhere and seem to want to give you a ride.
It’s in Tanith Lee, I believe… "Go nowhere on a horse that fades."
Two sentence horror stories.
Using some of these to spur my creative juices.
some gave me unpleasant chills.