Pour boiling water on gelatin in large bowl; stir until gelatin is dissolved.
Stir in reserved beet liquid, the beets, celery, onion and horseradish.
Chill until thickened slightly but not set; pour into 4 cup mold or 6 to 8 individual molds.
Chill until firm; cover. (Can be served immediately.)
At serving time, unmold Beet and Horseradish Mold on salad greens and serve with mayonnaise.