Final answer:
The houyhnhnms' nature and customs do not include being ruled by emotions.
Step-by-step explanation:
Out of the given options, the one that does not describe the houyhnhnms' nature and customs is option 4) they are ruled by emotion. The houyhnhnms, which are a fictional race of intelligent horses from Jonathan Swift's novel Gulliver's Travels, are known for their virtuous disposition, belief in friendship, and preservation of decency and civility. However, they do not exhibit emotions like humans do. They are portrayed as rational beings who rely on reason rather than emotion.