Scholastic Dragon Rider The Griffin's Feather by Cornelia Funke

The last winged horses are on the brink of extinction. Three foals lie curled in their eggs in a sanctuary for threatened creatures, where a young dragon rider lives with his silver dragon. The foals are ill, and the pair volunteer to seek the only cure: a Griffin's feather. But Griffins, with the heads of eagles and the bodies of lions, are a dragon's fiercest enemy, and live far across the world in the sweltering jungle.