Similar to the Greeks and Romans, the various Celtic tribes shared the basic premise of their mythology, but the names and deeds of the deities are different. Among these tribes, there are many goddesses associated with horses. The Cymric Celts have Rhiannon, the Gaels, Etain Echraidhe, and Macha, who figures prominently in Ireland. All of these horse goddesses are tied together by an association with a white horse (Macdonald 1). The white horse is the only beast that was allowed to travel freely to and from the Otherworld, a general term for the dwelling places of the spirits and gods (Ellis 177). There are many tales of these goddesses, and yet they are just taste of the rich flavor that is Celtic mythology.
Horses are celebrated by the Celts during Lughnassadh, the harvest festival. Initiated by the Gaelic sun god Lugh, who is honored for introducing the craft of horsemanship to the Celts, this festival featured horse racing and exhibition (Macdonald 1). Because horses were an integral part of farming, they were used by the Celts in fertility rituals. The adornments, for horses found by archeologists, are evidence of their reverent during these celebrations (Proinsias 51).
The mythological beasts the kelpie and the cailleach are also featured in Celtic folklore. The kelpie is a water spirit that takes the form of a young horse who willingly lets humans mount only to leap into the loch and drown them. "The Â‘cailleach', the old hag of the night who takes the form of the nightmare, the horse that is bringer of bad dreams and ill omen (Macdonald 2)." It is evident by these two creatures that horses had a negative aspect among Celtic mythology as well. The Celts had many animals specifically associated with a deity, but none seem to be as highly placed as the horse. This reverence exhibited itself in their artifacts, folklore, and influence on other cultures. It is evident by the Roman worship of the goddess Epona "The Divine Horse" that they were impressed by horsemanship of the Gaul (Proinsias 55). The Celts viewed the horse as more than a beast of burden, it was a direct link to the spiritual world.
I've included an Irish tale that illustrates the symbolism of the horse in Celtic mythology.
The Red Pony
A poor man had so many sons that he could not maintain them all; so the last one at the door coming home from school he decided to shut out. The lad went away and walked on until he came to a house on the side of a hill, where he was offered shelter for the night. In the morning the man of the house gave him a present, a red mare, with a saddle and bridle. It was a magic pony that could talk and travel over the sea (meaning it was an Otherworldly horse that could travel between the worlds). As he was riding along, the pony advised the boy not to touch anything he saw before him. Along the road the boy saw a box with a light in it and a lock of hair. Despite the warning, the boy took the box with him. (The light signifies illumination gained from knowledge; the lock of hair belongs to a beautiful woman. The story goes on to show that seeking knowledge has consequences that you must be prepared to follow through).
The boy went to work as a stable lad, with eleven other boys. While going out to the stables at night the boys noticed the bright light coming from the lad's stable, and told the king. The king had the box brought to him and found the lock of hair within, and said the lad must bring to him the woman to whom the hair belongs. (The king was always a Druid; he is setting the boy a magical task. It signifies that he must seek the Goddess in the Otherworld first).
So the boy and the red pony went over the sea. They saw the beautiful woman of the lock of hair, who asked for a ride on the pony, and the pony brought her back and took her to the king. Then the woman herself set a task for the boy (his second task, set by the Goddess this time). She told him that she would not marry any man unless he could fetch the bottle of healing water that was in the Eastern world. The pony told the boy that he must kill her in order to carry out the task. So the boy cut the pony open and out flew three ravens. (Three is a very symbolic number, signifying the three levels of body, mind and spirit, and also the Celtic number of completion. The story retains a sort of 'folk memory' of the ancient practices of divination from the entrails of animals and also from the flight patterns of certain birds). Two of the ravens went into the body of the pony and drank its blood, then flew out. When the third raven went in, the boy closed up the pony, and told the ravens he would not set their companion free until they brought the bottle of healing water for him. They came back that evening with the bottle and he let the other raven out. He poured the healing water over the pony and she became well again. (This signifies the Goddess ruling over Life, Death and Rebirth as she continually renews Herself each new moon).
The third task for the boy was to jump in and out of a barrel of boiling pitch without harming himself. He rubbed himself first with the healing water, and was then able to perform the task unharmed. He jumped in and out three times. Then he and the beautiful woman were married, and the wedding lasted three days and three nights. (This test shows the bravery of the warrior who must face Death on three levels and only then was accepted into Manhood). After the wedding, the lad found the bones of the mare and was rather distressed at first. However, it was explained to him that the woman and the mare were one and the same. This echoes the well known theme in legends and folk tales of the transformational nature of the Goddess, from an old hag in the evening who asks the warrior to take her to his bed, and in the morning has become a beautiful maiden.