fabulous monster of Greek mythology, slain by Bellerophon, late 14c., from Old French chimere or directly from Medieval Latin chimera, from Latin Chimaera, from Greek khimaira, name of a mythical fire-breathing creature, slain by Bellerophon, with a lion's head, a goat's body, and a dragon's tail; literally "year-old she-goat" (masc. khimaros), from kheima "winter season," from PIE root *gheim- "winter."
Supposedly a personification of snow or winter, but the connection to winter might be no more than the ancient habit of reckoning years as "winters." It was held by the ancients to represent a volcano; perhaps it was a symbol of "winter storms" (another sense of Greek kheima) and generally of destructive natural forces. The word was used generically for "any grotesque monster formed from parts of other animals;" hence the figurative meaning "wild fantasy" first recorded 1580s in English (13c. in French).
Beestis clepid chymeres, that han a part of ech beest, and suche ben not, no but oonly in opynyoun. [Wyclif, "Prologue"]