The Collaborative International Dictionary
Zoo- \Zo"["o]-\ A combining form from Gr. zwo^,n an animal, as in zo["o]genic, zo["o]logy, etc.
Douglas Harper's Etymology Dictionary
word-forming element meaning "animal, living being," from comb. form of Greek zoion "an animal," literally "a living being," from PIE root *gwei- "to live, life" (source also of Greek bios "life," Old English cwicu "living;" see bio-).
pre. Forming compound words pertaining to animals.