Whether PTSD existed in the past, and especially the ancient past, is an extremely controversial topic. Some scholars maintain that the nature of pre-modern warfare was radically different, to the point where things like "shell-shock" could not occur; meanwhile, social and ethical structures prevented war as such from being a very traumatic experience. From this perspective, PTSD is ultimately a modern cultural phenomenon. Others argue that wherever humans have endured traumatic experiences, whether in war or in civilian life, PTSD will also have existed.

The trouble here - and the reason why we should be very careful to make any rash claims - is that post-traumatic stress disorder was only formally defined as a psychological disorder in the 1970s. For a medical professional it is neither ethical nor justifiable to retroactively diagnose people who can no longer be interviewed in person. Therefore, from a purely professional perspective, PTSD "didn't exist" before the 1970s. The symptoms are easily identifiable in the record of the wars of the earlier 20th century and beyond; scholars have even tried to find sample cases in ancient Assyria and Classical Greece. However, we cannot simply declare that these people suffered from PTSD. We do not know enough, and will never know enough, to make the diagnosis.

One way around this has been to establish the observable neurological changes that cause PTSD symptoms to manifest. These changes show that the disorder is not just a cultural construct, but a physical response to traumatic experiences. This evidence allows us to detach PTSD from its formal label and declare that it is indeed a universal human experience. Such evidence is more solid than the ongoing effort of scholars like Lawrence Tritle to trawl through the historical record of ancient wars looking for descriptions of people's behaviour that sound like what we might expect in PTSD sufferers.

There have been a number of threads on this topic on AskHistorians, some of which are listed in the FAQ. In this thread, I discuss the Greek evidence and provide links to good answers on the Roman evidence. u/kookingpot tackles the Assyrian documents that are sometimes cited as evidence of ancient PTSD.

Our understanding of PTSD as it is today, relies a lot with our understanding of human psychology having developed over the course of time. Here is a brief article I wrote on the subject that illustrates the development of understanding and treatment of psychological illness from WWI forward.

The point being, that it is only due to the advances in medicine over the last century that have allowed us to more comprehensively comprehend the effect war has on the psyche. This indicates to me that the prevalence of PTSD would be the same throughout history (as the same conditions which cause it are extant) though it would not be understood as such. In an age where leaches and bleeding were common go-to's for physical illness, we certainly should expect medicos of the day to have a nature of understanding of mental health at a comparable level. Look for words along the lines of "melancholy" when describing the emotional state of soldiers in the pre-industrial age.

This video gives some interesting insight to how Marshal Ney might have been under duress of PTSD symptoms during a crucial moment at the Battle of Waterloo.