Tales of the Mabinogion
Olwen of the White Track
Olwen was the daughter of Hawthorn, king of the race of giants. She was so beautiful that any who beheld her was filled with love. Wherever this maiden stepped, four white trefoils grew, which is reflected in her name - Olwen meaning "she of the White Track".
The hero Eilhwch decides that he will find and wed this beautiful maiden, despite the warning that no one ever returned from such a quest alive. In order to win his love, Eilhwch is set a series of heroic (read impossible) tasks by Olwen's father. [Note the similarity to the Hercules myth - that hero was also set twelve seemingly impossible tasks to perform, which he proceeded to do].
Pwyll and Rhiannon
Pwyll, Prince of Dyfed, took a fancy one day to sit on a mound at Narberth (North Pembrokeshire) which was said to presage strange adventures. Sure enough, he soon saw a lady dressed all in gold approaching upon a white horse. He sent his servants to fetch her, but no matter how fast they rode, she somehow managed to keep the same distance ahead of them.
Pwyll decided to pursue her himself, but again, no matter how fast he rode, he was unable to catch up to her. Finally, he called out to her, telling her that he loved her. Instantly she stopped, declaring with some humour that "it were better for the horse" that he called out sooner. The Lady Rhiannon said that her family was forcing her to marry someone against her wishes, but now she would have Pwyll or no-one. Sure enough, after many more adventures, they were wed.