Pangur Bán Celtic Fantasies Series by Fay Sampson

“Now! Now you shall see what happens to those who would break the spell of mermaids!” Deep down in the underwater kingdom, Princess Finnglas is in the deadly grip of an evil sea monster, and the mermaids have bewitched Niall. Pangur Bán, the white cat, is desperate. He must rescue them—but how? Only Arthmael can do it. But who is Arthmael? And where is he? Can Pangur Bán find him in time?

This series follows the exploits of the white cat, Pangur Ban, his friend Niall and the Princess Finnglas. In the second book in the series, the princess is on a quest to find her beloved lost horse, but this is only the beginning of a great adventure and a continuing struggle against dark forces. Other titles in the series: Pangur Ban, the White Cat Finnglas and the Stones of Choosing Shape-Shifter, the naming of Pangur Ban

"On, Cloud–Clearer, on!" screamed Princess Finnglas. But she knew her horse was already pouring out all his strength. And the rival black stallion was still ahead. Finnglas is riding for her life and the crown. If she loses, she will die and the Summer Land will fall into the grip of evil powers. But this race is only the first of the Seven Trials. There are more terrifying tests ahead.

Deep in a mountain cave a witch is plotting mischief: "We need something small, something sly, to carry a spell . . . and then we shall see who reigns on the Black Mountain!" The kitten Shape-Shifter is her victim. But before the charm is complete, the kitten escapes. He flees, in a terrifying chase, caught in a spell gone wrong and a shape that he cannot control. He brings disaster even to those who want to help him. Is there another power that can break the spell?

Under the spell of the evil Rhymester the once peaceful kingdom of Senargad has become a place of fear, death, and destruction, where the savage Wolf-Guard roam the country and prisoners are held captive in subterranean caverns by the terrible Serpent of Senargad.

Princess Finnglas and the white cat Pangur Báan set out to recapture an unbroken white colt which, according to legend, may unwittingly force their people into battle.