This is "The Appaloosa of Alamosa."
"When that Appaloosa mare went out to play,
freckles on her saucy rump, sun on her mane,
no one knew Snippy was fated to see
plain little green men descending
from the sky on silver saucers.
What happened to the horse is an unkempt question
in the universe.
I've known ladies with cantilevered tongues
to swear that they were set upon by hordes of sky men
who spoke syllabic words like Eskimo.
And stethoscope the songs of the silly goose girl
beating of their hearts that came and went
in flirting fits and starts.
But the Appaloosa lost her head and eyes.
Her bones changed color, white to red to blue.
The maggots would not touch her, that horse who
a stopwatch clocking her to paradise
was lured by summer men from a silver cigar.
The story is incomplete and stops in air."