It is a small hardcover book with just one short story in it. There is a young lady who is gifted, I don't remember the extent of her gift. But she helps a river horse (a pooka?) and he feels indebted to her. Something happens to her and he does something about it. I believe it was published sometime in the 1990s. There were several books in, what I believe was a series, the stories were by different authors. I can remember the title/author of another one (an exchange of gifts, by Anne McCaffery), but I have been unable to find the one I'm looking for.SPOILER (view spoiler)[he gains a human form to rescue her and at the end they are both swans/humans (hide spoiler)]
Fifteen-year-old Ariella, lady-to-be of medieval Swan Manor, possesses magical healing abilities that she practices on the animals in the forest adjacent to her father's lands. One day a magnificent black horse emerges from the nearby river in need of her ministry. The horse is Merod, and he is a more-than-mortal Kelpie, a magical being who converses telepathically with Ar Fifteen-year-old Ariella, lady-to-be of medieval Swan Manor, possesses magical healing abilities that she practices on the animals in the forest adjacent to her father's lands. One day a magnificent black horse emerges from the nearby river in need of her ministry. The horse is Merod, and he is a more-than-mortal Kelpie, a magical being who converses telepathically with Ariella. Distrustful at first, he warms to Ariella gradually, which is vital to her when, after her father's sudden death, she is taken away by a brutish cousin to be his bride. Prolific fantasist Lackey deviates from her 400-plus-page norm to write a story that, in development, tone, and scale, harks back to the fairy tales of George Macdonald. If it doesn't match the charm of those Victorian gems, it is agreeable enough, perhaps best in the descriptive passages about the harvest at Swan Manor and the journey of Ariella's virtual abduction by her cousin. ...more