Snowflake

"I would save my horse, Coalblack, and the Witch's Snowflake (a noble beast and worthy of a better mistress) which are both stabled in the courtyard."

- Rilian

Snowflake was the name of the horse owned by the Lady of the Green Kirtle during her reign in Underland. She was a dumb mare (as distinct from a Talking Beast).

Physical Description
Snowflake was a white horse, "so lovely that you wanted to kiss its nose and give it a lump of sugar at once." 2

Role in the History of Narnia
The Lady of the Green Kirtle rode Snowflake during her many outings to Overland with the enchanted Prince Rilian, which she made in order to accustom his eyes to the sunlight before their invasion of Narnia. The Prince rode Coalblack, a black horse who was also owned by the Witch. Most of these outings likely occurred near Ettinsmoor and Harfang. It is also probable that the Witch rode the mare on journeys throughout Underland.

After the disenchantment of Rilian and the death of the Witch, Eustace and Puddleglum rode Snowflake to the nearest tunnel between Underland and Overland, while Rilian and Jill Pole rode Coalblack. The Sunless Sea was flooding quickly throughout Underland because of the death of the Witch, which ended her magic, so it is unlikely that Rilian and his three rescuers would have escaped without the horses' aid.

Eventually, the roof of the tunnel became too low for them to pass through on horse-back, so the group had to dismount and lead the horses. When they reached the end of the tunnel, they were rescued by a group of Fauns and Dwarfs, who were in the middle of performing the Great Snow Dance.

Little else is known for certain about the two horses, but it is most likely that both stayed happy and well cared for in Narnia for the rest of their lives, possibly in Cair Paravel.

For a fuller explanation of Rilian's captivity and enchantment, see The Silver Chair.