Agrippa d'Aubigne (1552-1630), a soldier and poet best known for the poem Les Tragiques, was part of the power behind the rise of Henri de Navarre to the French throne (as Henri IV). But his loyalties were put to the test by Henri's character, which d'Aubigne came to view as "a mean, fickle, envious, ungrateful, and treacherous prince, turning even his old spaniel Citron adrift to die : "Payement coustumier du service des rois !" (Customary payment for service to kings!) as D'Aubigne wrote in a fine sonnet, which he tied to the dog's neck on sending it back to its master." (Quote from the biographic entry on d'Aubigne in the 1842 edition of The Biographical dictionary of the Society for the Diffusion of Useful Knowledge; link's here, you'll have to dig).
Rogier Grenier, in the previously-mentioned The Difficulty of Being a Dog, tells a little more of the story. Around the same time he resigned for a while from Henri's service, d'Aubigne was passing through Agen when he found Henri's spaniel Citron, abandoned and starving, where in happier days he had been used to sleeping in the king's bed. D'Aubigne found the dog a foster home, and had a poem attached to the dog's fur which pithily listed the dog's reversal of fortunes, ending with the punchline above.
The very next day, Henri came through Agen, and the townspeople brought Citron out to show him. He read the poem - "and blanched," Grenier writes. (Page 38.) At least he had some shame.