I'm not convinced they are intended as sphinxes, prudence. Though, I think it was le pendu (or perhaps myself) who pointed out either in a thread on Aeclectic or elsewhere, that the Vieville carver (if there was a single person who drew the whole deck) knew how to draw horses: look at the court cards - so there does appear to be some rather bizarre depiction here.
It could be, however, that in this case the sense of Plato's three parts of the soul did play in a little in rendering the heads more human-like... it would be nice to know what Vieville had in mind, or why the depiction is as it is (it could after all simply be that the person who drew the design for the carver did it poorly, and that it was further eroded by the hand of the carver).
Which brings me back to OnePotato's response above, viz:
If the complete integrity of all of the details of the image was seen as important to its "function" (as a symbol) in the first place, why would so many of these artists and artisans crop with such abandon? Were they all ignorant of content, and lacking the drive to get it right? or is it not possible that the "sloppy" bits were not important, and they well knew it?
I do think this hints at a possibly fundamental issue that over time diminished in importance: the details of the image as symbol.
After all, if the deck was produced for the game, then those images could indeed lose their attention to detail because indeed, as quoted, "the "sloppy" bits were not important, and they well knew it" - not important for the players, as long as familiarity with the general form of the design stayed. Hence we also begin to see double-headed trumps - certainly something that shows that 'details' which we
consider important were not deemed to be so by those card makers and those who used them.
This does not mean that very early models lacked symbolic content. It does show that these, even within a couple of centuries, were deemed of less value than a playing deck players could easily use and identify distinctly one card from another.
Perhaps calling the design "sloppy" is not so much a 'projection of contemporary sensibility', but rather a description that recognises that details that had earlier been carefully drawn became aesthetically less important, and symbolically the poorer as a consequence of its uses. This does not mean that some card makers did not attempt to re-establish very careful and aesthetically pleasing decks - whether it be with the Tarot de Marseille-II
or the numerous double-headed post-revolutionary designs.
I suppose I do not have the view that the card-makers were generally 'initiates' who primarily sought to pass on symbolic content. Rather, they appear to have generally copied one from the other in order to produce decks for a market that appears to have been reasonably large. The quality of the paper (or rather, card-stock), and the basic design, would, I suspect, have been as important as getting the deck image design 'right'. And this last thing - the quality of card-stock, is something that, as OnePotato and others who have held 18th century decks, is quite surprising at first (or perhaps it just was for myself when I first held a Dodal).
If the deck used for gaming has good card-stock, and its imagery sufficiently standard, then a card-maker would have a sellable product. Over time, the need to pay attention to symbolic detail becomes, in terms of gaming, eroded. And if nothing else (and of course there is much else), this important aspect of the deck as game is something that Dummett has brought back to conscious memory.