Jesus was calling Herod a fox. A fox is a small wild dog. Possible meanings are 1) Herod was not much of a threat at all 2) Herod was deceptive. (See: [[rc://en/ta/man/translate/figs-metaphor]]).
The Jewish leaders claimed to serve God. And yet their ancestors killed many of God's prophets in Jerusalem, and Jesus knew that they would kill him there too. AT: "it is in Jerusalem that the Jewish leaders kill God's messengers" (See: [[rc://en/ta/man/translate/figs-irony]])