The Mystery Of Plum Park Pony (1980)

The Mystery of Plum Park Pony
Lynn Hall, il. Alan Daniel
1980, Garrard Publishing Company

It was heaven, Susan thought, to ride once more.  The ponies trotted happily down the gravel roads and paths of the park.  They seemed to enjoy the riding as much as Susan and Kent did.

Susan and Kent live across the street from Plum Park, a small amusement park.  In addition to the Tilt-O-Whirl and the roller coaster, the park attractions include a pony ride.  On this spring day, Susan and her pal have followed up on the pony rider operator's promise they can exercise the ponies before opening day.  Susan is puzzled that there's an extra pony in the herd, and the kids soon discover that the newcomer is a runaway from a nearby show barn.  By the time they're figured this out, the runaway has vanished, and the tracks lead to the Tunnel of Terror.

There was no light anywhere.  Susan fell over an electric cable on the ground.  Kent ran into a corner of a scary scene.
"I don't know about you," Kent said, "but I'm lost."
Suddenly there was warmth and movement beside them.  Susan could hardly see the outline of the pony.  She she knew the pony was standing very still.
"I think she's caught on the track," Susan said.

A short tale for beginning readers, with very 1970's illustrations. 

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Other editions
It was reprinted in paperback as The Mystery of the Phantom Pony by Random House, il. Marie DeJohn, cover by Ruth Sanderson.