EQVVS OCTOBER (October 15)

On the Ides of October (October 15), the ancient Romans brought an important sacrifice to Mars, the God of war and agriculture on the Campus Martius ("Mars field") [i]. The victima (sacrificial animal) was the Equus October (“October horse”).


On this day, there was a race with bigae (two-horse chariots) and it was the right-hand horse of the winning chariot that was chosen for the sacrifice. 


The horses traditionally ran counter-clockwise in Roman chariot races [ii]. This means that the right-hand horse was on the outside and had to run faster than the one on the inside because it had to complete a bigger circle. The right-hand horse of the winning chariot was therefore considered to be the best horse in the race, making its sacrifice the best shot at trying to please the mighty Mars Pater.


The October Horse was killed with a spear [iii]. Usually, an axe or mallet was used to kill sacrificial animals, but as Mars is often depicted carrying a spear, using one for this sacrifice seems appropriate.

When the animal had been killed at the site of the race (the Campus Martius), the tail was quickly cut off and carried to the Regia (office of the pontifex maximus, the high priest of Roman state religion). At the Regia, some of the blood would be spilled on an altar [iv].

The head was also cut off and competed over by the residents of the Via Sacra and the residents of the Subura. As a reward, the winners could display the head in their neighbourhood.


If the residents of the Subura won, they would hang up the head at the Turris Mamilia and if the residents of the Via Sacra won, the head would follow the tail to the Regia to be hung up there [i].
At some point during the ritual, the head was adorned with bread or cakes [v].


Even in antiquity, there were several theories as to the origin of this ritual [iii], but it seems to have been a celebration for the end of the harvest [v], as well as the official ending of the campaigning season [vi].

References:
[i] Festus, 190. 
[ii] Potter, David Stone & Mattingly D.J. (1999) Life, Death, and Entertainment in the Roman Empire. University of Michigan Press: p237.
[iii][Timaeus, as quoted by Polybius. Histories 12.4.  
[iv] Plutarch, Roman Questions 97.
[v] Festus, 246. 
[vi] Eckstein, Arthur M. (2006) Mediterranean Anarchy, Interstate War, and the Rise of Rome. University of California Press: pp. 205–206.