Remember the story Ms. Faragonda tells the Winx in episode two, “L’Albero della Vita” (The Tree of Life)? The fight between the Fairies of Earth and the Wizards of the Black Circle dates back to the age of King Arthur, or, as the Italians would say, Re Artù. How do we know? The slide showing a hand raising a sword out of the water is an allusion to the Lady of the Lake, who gave Arthur Excalibur in some versions of the legend. And Morgana the Fairy Queen is named after another character in the myths: Morgan le Fay. (Le fay is based the French la fée, which means “the fairy.”)
So if the two stories are related, could Artù’s name be important? Maybe Rainbow was just sticking to the King Arthur theme, and Roxy gave her dog that name without knowing the significance. Or Artù could be him reincarnated—the Wizards cursed him, and Roxy, being the fairy of animals, has the power to make him human again.
Anything’s possible in fantasy!