Changespell

She wasn't always a lady - Once she was the magnificent Dun Lady's Jess - until a malfunctioning spell not only transformed her from her world to ours but into human form in the bargain. After amazing adventures here she had been returned to her own world of Camolen, where she is sometimes a human called Jess and sometimes a horse called Lady. Her innermost self revels in her dual existence, but still she must come to terms with the strangely mixed feelings she is experiencing toward her Rider, a man she has love since she was a foal - and now loves as a woman. But other changes are afoot in Jess's world: previously her were-horse experience has been unique, but now it seems someone is changing horses and other animals into humans by design, a very sinister design... (from the back cover of the Baen Books first edition)