In all Marseille-style decks of which I am aware, the Moon card feature two dogs or jackals: a light blue one on the left, and a pink or buff one on the right. (See "The red and blue jackals.") Aside from the color and a slight difference in size (with the larger, blue dog perhaps corresponding to Sirius in Canis Major), the two animals are the same.
This color scheme, universal among Marseille decks, was jettisoned by the early Marseille-influenced esoteric Tarots, beginning with Oswald Wirth's 1889 deck. Wirth's canines are black and white rather than red and blue, but they still both appear to be dogs.
The 1909 Rider-Waite deck uses yet another color scheme, but also introduces more significant differences between the two animals. One has the floppy ears of a domestic dog, while the other has the erect ears and bushy tail of a wolf. Waite confirms this in The Pictorial Key to the Tarot, writing, "The dog and wolf are the fears of the natural mind in the presence of that place of exit, when there is only reflected light to guide it."
This dog and wolf theme is also used -- and rather more competently executed artistically -- in the 1929 Knapp-Hall deck. One animal is clearly a wolf, while the other is a domestic dog complete with a collar.
Mary K. Greer connects this dog-and-wolf theme with the French idiom entre chien et loup, meaning "at twilight." This is an expression of Latin origin (inter canem et lupum) -- with twilight being thought of as "between" the domain of the diurnal dog and that of the nocturnal wolf, or perhaps between the time when people put out their dogs to keep watch and the time the wolves begin to come out. French etymology dictionaries, though, give a different explanation: that it refers to the dim light in which it is difficult to distinguish a dog from a wolf ("l'heure où la lumière décline car on confond alors facilement entre chien et loup"), and it is apparently in this sense that most modern Francophones understand the idiom. (Inter canem et lupum is also sometimes used in the sense of "between a rock and a hard place," but the obvious contrast between man's best friend and his deadly enemy makes it hard to see this as anything but an ignorant corruption.)
Anyway, this Latin and French idiom never made it into English, so it is curious that the French and French-Swiss decks have two dogs, while the English and American decks introduce the entre chien et loup imagery. However, there are precursors in the French literature (with which Waite, having translated much of it, was quite familiar). Wirth's Le tarot des imagiers du moyen-âge (1927) describes the two animals as "the big black dog" and "the little white dog," with no hint that either of them might be a wolf. However, when the opening paragraph of his description of the Moon card is read with the idiom in mind, it is hard to deny its relevance.
In order to display the splendours of the sky, the Night plunges the earth into darkness, for the things above are not revealed to our sight except to the detriment of those below. However we aspire to relate the celestial to the terrestrial by a simultaneous contemplation, which is made possible when the Moon spreads her pale light. The body which is close to the stars without subduing their brightness completely, only half lights up the objects bathed in her uncertain and borrowed light. The Moon does not allow us to distinguish colours; everything her rays strike upon she tinges with a silvery grey or with vague bluish shades, leaving the opaque darkness of the shadows of night to continue.
Papus did not produce a deck of his own. In his Le Tarot des Bohémiens (1889), the section on each trump has two illustration: the 1889 Wirth card and the traditional Tarot de Marseille. Although both of these show two dogs, Papus refers in his commentary to a dog and a wolf.
In the middle, a dog and a wolf howl at the moon, a crayfish comes out of the water and crawls in the midst of these animals. The entry of the Spirit into Matter is a fall all the greater as everything conspires to increase it. The "servile spirits" (dog), the "ferocious larvae" (wolf) and the "crawling elementals" (crayfish) are there who watch for the fall of the soul into matter to try to oppress it even more.
Wirth hints at the idiomatic meaning of entre chien et loup but identifies both animals as dogs. Papus identifies them as a dog and a wolf, despite using cards that portray them both as dogs, and yet his commentary on the card says nothing about twilight of the difficulty of distinguishing things in half-light.
One suspects that the "missing link," or rather the "common ancestor," is Wirth's first Tarot book, Le Livre de Thot comprenant les 22 arcanes du Tarot (1889), but unfortunately I have not yet been able to track down a copy to check. My hypothesis is that this earlier book explicitly refers to the dog and the wolf; that his later book, with its emphasis on astrology, dropped the wolf references in favor of Canes Major and Minor; but that traces of the author's earlier emphasis remain in the introductory paragraph. If I ever do manage to get my hands on Le Livre de Thot, it will be interesting to see if this speculative reconstruction is borne out.
