From the chapter on "Pets in Literary Life," from the 1892 book by Eleanor Lewis, Famous Pets of Famous People . . .
The pets and authors of the past may be briefly glanced at on our way to those of to-day. We may begin with the learned Justus Lipsius, erstwhile professor at Louvain. This worthy went daily to his lecture-room with a retinue of dogs, whose portraits, each with a commemorative description, adorned the walls of his study. Three have been individualized for posterity as Mopsikins, Mopsy and Sapphire.
Tarot, Franza, Balassa, Ciccone, Musa, Mademoiselle and Monsieur, were, in their long-vanished life-time, companions to Agrippa, the astrologer and scholar. The knowing little Monsieur was permitted, as special favorite, to sleep upon his master's bed, eat from his plate, and lie upon the table beside his papers, while he wrote. He may even have suggested to Goethe the black poodle in Faust, since, like Rupert's hound Boy, and Claver's battle-horse, he was commonly supposed to be a fiend.
Justus Lipsius was a Belgian humanist and intellectual.
Agrippa was Heinrich Cornelius Agrippa, who was also a solider and physician, and in 1529 wrote the Declamation on the Nobility and Preeminence of the Female Sex. Apparently Monsieur very likely was the inspiration for part of Goethe's Mephistopheles.
Rupert was Prince Rupert of the Rhine, and Boye was his badass poodle. Really.
Claver seems to be John Graham of Claverhouse.