"The head of a fish begins to stink first." Used of bad rulers, whose contagion poisons the rest of the people. The expression seems to derive from the language of common people.

[Piscis primum a capite foetet ... Piscis a capite primum incipit putere. Dictum in malos principes, quorum contagione reliquum vulgus inficitur. Apparet ab idiotarum vulgo sumptum.]

• "Fish start to stink at the top: [this is a proverb] applied to people who have scoundrels for leaders." [tr. @sentantiq]
• "The fish always stinks from the head downwards: The freshness of a dead fish can be judged from the condition of its head. Thus, when the responsible part (as the leaders of a country, etc.) is rotten, the rest will soon follow. ἰχθὺς ἐκ τῆς κεφαλῆς ὄζειν ἄρχεται, a fish begins to stink from the head." -- Jennifer Speake, ed., Oxford Dictionary of Proverbs (2015) [Source]