What is the book about a half-girl, half-monster?

Question

I am trying to find a children's book that I read in the '90s. It was sort of a frog princess story mixed with Dr. Jekyll and Mr. Hyde. From what I remember, a princess was banished to a pond (by her jealous sisters?) where she was taken by the creature that lives in the pond. They had a child who formed in a flower. A lonely viking woman found her and raised her as her own. The girl was beautiful by day with a horrible temperament, but by night resembled the monster (I remember the illustration of her looking like a frog) and had a meek disposition. In the end she helped a prisoner escape (at night in her creature form). They rode into the forrest on a horse. I remember her digging, but that is all I can remember.

Please help!

Answer 1

try looking for it on amazon and barnes & noble. when you see the title, it will probably ring a bell.