`WHAT if ...?'' is the tantalizing question that fantasy asks. Sometimes the author proposes a small alteration in life as we know it, other times the creation of a completely new world. In either case, a fantasy worth its salt will say as much about our own lives as it does about magic talismans and wizards.
Two of this season's fantasy novels for children use the same device - a toy horse - as the object that brings magic into otherwise ordinary lives.
In Marion Dane Bauer's Touch the Moon, illustrated by Alix Berenzy (Clarion Books, New York, $11.95, 96 pp., ages 9 to 12), 11-year-old Jennifer dreams of owning her own horse. When her birthday present turns out to be riding lessons and a ceramic horse, her disappointment is complete. She drops the figurine - but instead of breaking, it turns into the palomino she has longed for.
Coping with a full-size and independent-minded animal is far more difficult than Jennifer ever imagined. (And this horse can talk back!) Still, she is determined to learn to handle Moonseeker and take him for a nighttime ride.
Bauer's prose is clear and vigorous, whether she is writing about the delicious anticipation of birthdays or the terror of an animal trapped in a cave. Even the youngest reader will not miss the point when Jennifer tells Moonseeker, ``Real life is what you can really do. Pretend is just what you wish you could do.''
The story is not simply a lesson in responsibility and reality. By the time she gets safely home, Jennifer has reaffirmed the importance and the glory of ``reaching for the moon.''
The child, rather than the toy, is transformed in Ann Rabinowitz's Knight on Horseback (Macmillan, New York, $13.95, 76 pp., ages 8 to 12). While traveling with his family in England, Eddy discovers a tiny horse and rider in a London antiques shop.
Once he touches the horse, Eddy's perceptions change. At the Tower of London and again at Bosworth Field, Eddy experiences visions that reveal the private life of Richard III. The ancient toy links Eddy to a shadowy, cloaked figure that no one else can see. And at a performance of ``King Richard the Third,'' Eddy finds himself loudly defending the king whom Shakespeare portrays as evil.
Eddy is a more complex character than the straightforward Jennifer of ``Touch the Moon,'' and his story is more subtly told.
At 13, Eddy is on an emotional roller coaster. Small and chronically ill, he struggles for independence from his stifling family, all the while desperately striving for his father's affection.
What if another misunderstood figure could take Eddy out of his uncomfortable present and give him the attention he craves? Eddy is drawn deeper and deeper into the personal life of the tragic king until the boy must make a very difficult decision. Rabinowitz's sensitivity to adolescence and her personal approach to history make this a compelling and thought-provoking fantasy.
Grace Chetwin has created a completely new world for her four-book series, ``Tales of Gom.'' Like Ursula LeGuin's ``Earthsea'' trilogy, Chetwin's story is an extended epic, patterned on the hero myth. ``Gom on Windy Mountain,'' which appeared last year, introduced Chetwin's appealing young hero who talks with animals and asks advice of the wind. Gom's mountain world is convincingly portrayed, from the personality of Sessery, the cave breeze, to the pattern of veins on a leaf that Gom treasures.
This year's installment, The Riddle and the Rune (Bradbury Press, New York, $13.95, 256 pp., ages 10 and up), takes Gom on a quest to find his mother, a powerful wizard of the Brown Order. In the process, Gom learns his purpose in life.
The story is rich with mythic elements: a prophetic riddle, a dragon's hoard, animal friends, and a battle with evil incarnate. Chetwin's inventions include Ganash, a helpful plesiosaur-like creature that turns into a dragon if kept out of the sea too long, and the cito, a magnificent horse born once every thousand years.
Those who have not read the first book will find Gom's character insufficiently developed here, however. And the plot's strengths are undermined by two last-minute rescues based on coincidence alone. It's as if Chetwin, eager to get on to the next two books in the series, did not give the answer to her ``What if ...?'' the full attention it deserves.
Carolyn Polese's most recent book for children, ``Promise Not to Tell,'' received a 1986 Christopher Medal.