Using a paraphrased OED definition, a fable is a short story devised to convey some moral or lesson, usually using animals. Aesop used this form in the famous Aesop's Fables to make a comment on human nature.
In Gulliver's Travels, Jonathan Swift uses the fable's elements to show us that human nature is neither purely reason-based nor instinct-based. He presents the Houyhnhnms as an example of creatures driven wholly by reason and, in contrast, the Yahoos as creatures driven wholly by instinct. Swift points out the ridiculousness of Gulliver's thinking: Gulliver believes that the Houyhnhnms are perfect in their emotionless logic, but in striving for rationality, Gulliver degrades himself into a prime example of irrationality. The Houyhnhnms, in line with Aesop's tradition, are actually talking horses -- and Gulliver tries to imitate them by mimicking their speech patterns, etc. which are all undeniably horse-like; he has lost all sense of what constitutes logic.
Further, Swift's purpose is to attack an over-reliance on reason, by portraying the consequences. The Houyhnhnms lack passion whatsoever. For marriage prospects, they check teeth, giving no consideration to love but only to good physical breeding, and they commit genocide for the greater good.
- Deborah Kim, 1B