I believe it is a bull's head that the King of Pentacles is stepping on. Although it could be a gnome's head - which is appropriate for the role of the King in keeping these forces subservient.
https://marygreer.files.wordpress.com/2015/09/gnomes-head2.jpg
Some of the images in the cards come from the so-called "Westcott Court Cards" - found among his papers, although they may have been drawn by Moina Mathers. On the back of each drawing was found a written description that included an "ornament" for each card's symbols that were to appear in the illustrations. [I haven't included the details of the sceptres.]
http://www.amazon.com/Golden-Dawn-Court-Cards-Studies/dp/1558183361
I highly recommend Darcy's book. Unfortunately I don't have permission to reproduce any of the cards, but I will list the "ornaments" - some of which appear around the neck of the RWS Court Card figures, although without the wings, and elemental symbols that were to appear on other parts of the card. The Thoth deck is more true to the descriptions.
Wands - Fire - Fixed Fire = Leo/lion
King of Wands - winged lion's head
Queen of Wands - winged leopard's head
Knight of Wands - winged black horse's head
Knave of Wands - tiger's head
Cups - Water - Fixed Water = Scorpio/eagle
King of Cups - eagle open wings - lotus, serpent (RWS-fish around his neck)
Queen of Cups - ibis open wings - crayfish (RWS-winged bird on side of chalice)
Knight of Cups - peacock open wings - white horse, crab
Knave of Cups - swan open wings - dophin, turtle
Swords - Air - Fixed Air = Aquarius/human head
King of Swords - winged angel's head - arch-fairies
Queen of Swords - winged child's head - decapitated man's head, cumulous clouds below
Knight of Swords - winged hexagram - brown horse, driving clouds
Knave of Swords - Medusa's head - grey smoke from silver altar, cirrus clouds
Pentacles - Earth - Fixed Earth = Taurus/bull
King of Pentacles - winged bull's head - bull
Queen of Pentacles - winged goat's head - goat, barren land (not so in RWS*)
Knight of Pentacles - winged stag's head - light brown horse, corn field
Knave of Pentacles - winged ram's head - grass, flowers, grove of trees
*Waite and/or Smith obviously felt that the Queen of Pentacles should be in a very fruitful environment rather than a barren one!
Also, notice that the color of the RWS horses seems most appropriate to the elements.