In 20,000 Leagues Under the Sea, what was Monsieur Pierre Aronnax's opinion about the mysterious creature in the sea, and what name did he give to it?

The greatest depths of the ocean are totally unknown to us. What happens there? What beings can live twelve or fifteen miles below the surface of the sea ? We can scarcely conjecture . . . (spoken by Professor Pierre Aronnax) (part 1, chapter 2, Twenty Thousand Leagues Under The Sea)

Professor Pierre Aronnax, a former medical doctor and now Professor of Marine Biology at the Paris Museum of Natural History, is the narrator of Jules Verne's Twenty Thousand Leagues Under The Sea. Professor Aronnax heads the expeditionary team aboard the American frigate Abraham Lincoln, which has been tasked with tracking down the mysterious sea creature (which some call a sea monster) that has been attacking and sinking ships all over the world.
Professor Aronnax formulates a hypothesis as to what this sea creature might be:

"Therefore," I wrote, "after examining these different hypotheses one by one, we are forced, every other supposition having been refuted, to accept the existence of an extremely powerful marine animal" (part 1, chapter 2)

Professor Aronnax concluded that the sea creature was either previously undiscovered and therefore completely unknown, or it was some kind of "colossal sea unicorn" or "giant narhwal"—a member of the whale family, with a " king–sized tooth as hard as steel" that projects forward from its upper jaw, giving it the appearance of a sea-going unicorn (part 1, chapter 2).
Later in the sea voyage, Professor Aronnax remarks,

"Hmm!" I said to myself. "A cetacean as powerful as a whole cavalry regiment—now that's a whale of a whale!" (part 1, chapter 6).

Throughout his narrative, Professor Aronnax consistently refers to Nautilus as "the monster."