This is a very, very interesting question with few sources mentioning it directly but much more can be observed from the lives of individuals. I will try to give you a short overview just from top of my head and an useful link to a blog of a classical archaeologist and historian who researched extensively on this subject.

The earliest known mention of what could refer to PTSD are mesapotamian historical sources which point that soldiers often have nightmares about people they killed and atrocities they witnessed. Not understanding what these psychological issues actually were, they reasoned that ghosts of the enemies were possessing them out of vengeance.

Then we have one of the most important literary works of the Western civilization, the Illiad. It is filled with numerous mentions where soldiers and kings suffered mental breakdowns (Ajax killing himself, Achilles being prone to rage and traumatized by grief) and general mental instability which led to brawls and discord.

Ancient historians mention many famous generals and ordinary soldiers (Alexander the Great, Gaius Marius) exhibiting behavior which could be a sign of PTSD. These included anxiety, nervousness,loss of sight, insomnia and nightmares which were often dealt with consumption of alcohol. We can safely assume that with closer research of sources and biographies, more cases would be found.

One instance of how cruel can be the judge of history and historians about a person because of ignorance in the field of human psychology is the 1st century AD Roman emperor Tiberius. Tiberius was the best general of Augustus and one of the greatest Roman conquerors who dealt in person with many situations which posed danger to the early empire. He was one of the rare generals who was successful in Germania, crushed the Illyrian uprising, conquered Raetia and Pannonia.

In contemporary sources, his early years are praised as an example of a modest and good individual. Before Tiberius became a general and an emperor, he was a virtuous and merry person who worked diligently in lower offices and higher of Roman state administration. After ascending to throne he is depicted as dark, reclusive, nervous person and by Pliny the Elder called "the gloomiest of men".

After many, many wars in which he participated as a commander, was thrown back and forth in succession line, exile, separation from his only love, loss of his son and becoming an emperor, Tiberius is described as cruel,mentally unstable, dark and unjust individual who retired to island of Capri where he indulged in all sorts of perversions. Most of these assumptions are today proved incorrect and Tiberius is named as one of the more capable and just emperors.

The judge of contemporary historians on Tiberius is exceptionally negative but it is quite possible that his behavior was a result of PTSD caused by a number of big emotional traumas during his life.

In a tl;dr version of this, I would strongly argue that PTSD among people in antiquity and overall history was very common, from the lowest peasant with a pike to great statesmen who conquered almost all the known world. It would be very interesting to more closely inspect the sources and find more examples which point to it.

http://phdiva.blogspot.hr/2010/11/rage-of-achilles-and-ptsd-in-antiquity.html

u/Iphikrates recently shared some great insights on the topic of PTSD among soldiers of Greco-Roman antiquity and whether we can apply clinical terms like 'PTSD' to people of the past: Can anyone here recommend any works upon mental health throughout history, specifically concepts such as PTSD, depression etc. and especially as a consequence of war/conflict?

This is a questions that comes up every 3 to 6 months. I would suggesting searching the subreddit for detailed answers.

As a physician I would add that PTSD is a relatively recent diagnosis. As with all medical diagnoses, we can infer the possibility of it, but we have absolutely no way to comment accurately. PTSD first became a recognized diagnosis in 1980 in the DSM3.

The most current diagnostic criteria for PTSD are in the DSM5. They are (somewhat paraphrased):

1. Exposure to actual or threatened death, serious injury, or sexual violence in one of the following ways: Direct exposure to event, witnessing the event, learning of the occurrence of the event with a close family member or friend, experiencing repeat exposure to such events (ie first responders).

2. Presence of at least one intrusive symptom: recurrent, involuntary, and intrusive distressing memories of the event; recurrent distressing dreams related to the event, dissociative reactions such as flashbacks where the event feels as though it's recurring, intense or prolonged psychological distress at exposure to internal or external cues symbolizing the event, marked physiological reaction to said cues.

3. Persistent avoidance of stimuli.

4. Negative alterations in cognition and mood associated with the event.

5. Two or more of the following: irritable/angry behavior, reckless/self destructive behavior, hyper vigilance, exaggerated startle response, problems concentrating, sleep disturbance.

6. Symptoms of 2,3,4,5 for more than one month.

7. Distress causing distress and impairment in social, occupational, or other important areas of functioning.

8. Not attributable to a substance.

I hope that it's clear that retroactively diagnosing such from historical texts would be challenging and not truly diagnostic, merely suggestive.

Follow up. How did warrior cultures deal with what we know as PTSD? For example vikings thought death and battle was the best way out, did they get PTSD more or less easily?

Relating to u/invimo mention of PTSD in the Illiad, I would highly recommend the book Achilles in Vietnam: Combat Trauma and the Undoing of Character . The author compares the actions and emotions of Vietnam vets to characters in the Illiad. Super facinating and taught me a lot about how both ancient warfare might have been viewed by the average soldier and why Vietnam soldiers seem to suffer so greatly from PTSD.