The History of Animals (Greek: Τῶν περὶ τὰ ζῷα ἱστοριῶν; Latin: Historia Animālium) by Aristotle is a work of natural history which shaped the development of zoology for some two millenia.
Free online texts
Internet Archive: History of Animals, translated by Richard Cresswell. Multiple formats.
Internet Classics Archive: History of Animals, translated by D'Arcy Wentworth Thompson. HTML and text formats.
University of Adelaide (Internet Archive): History of Animals, translated by D'Arcy Wentworth Thompson. Multiple formats.
Wikisource: History of Animals, translated by D'Arcy Wentworth Thompson. Multiple formats.
History of Philosophy without any gaps: Classified Information: Aristotle's Biology - podcast by philosopher Peter Adamson.
Stanford Encyclopedia of Philosophy: Aristotle's Biology, by James Lennox.
Wikipedia: History of Animals.
The Great Conversation: Further reading at Tom's Learning Notes
Aristotle: On the Soul (De Anima).
Aristotle: Parva Naturalia.
Ancient Greek resources: Learn to read Greek classics in the original.