Lyman Frank Baum 
The Enchanted Island of Yew [EPUB ebook] 

Sesely, daughter of Baron Merd of Heg, and two companions are enjoying a picnic in the Forest of Lurla when they are accosted by a fairy. The fairy, bored with centuries of insipid fairy life, amazes the girls by pleading to be changed into a mortal. Though the girls are surprised that they might have the power to do such a thing, the fairy explains how it can be done. The girls agree to transform the fairy into a human boy for the space of one year. The newly-minted male is dubbed Prince Marvel, and, furnished with fairy arms and armor and an enchanted horse (a deer transformed), sets out to have adventures.