One of the oldest books about animals is the Historia animalium, by the Swiss naturalist Conrad Gesner (1516-1565). It's really a collection of five books, published from 1551 to 1558, that include animals both real and imaginary (e.g., unicorns and sea monsters). The image above is a page from volume 3, birds, and one of several scanned by the National Library of Medicine and now made available online along with other rare, historical texts from the collection.
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(Hat tip: Pharyngula)