DD is massively into horses Cod and has been since she was about 4. She also loves reading and there are loads of chapter books in a horsey series which would be lovely. Magic Pony is prob too young for her although this depends on her reading abilities.
Or even an Encyclopaedia of Horses and/or Care Book. DK do excellent books covering anatomy/care of etc
Failing that, ELC do playsets, and Grand Champions range in TRUS or CHestnut Ridge