PTSD has likely been a condition since the dawn of man. Of course you won't find anyone calling it PTSD until 1980 but the symptoms have been found in the accounts of soldiers for centuries. The oldest I know of:

1000 BC an Egyptian combat veteran named Hori wrote about the feelings he experienced before going into battle:

"You determine to go forward... Shuddering seizes you, the hair on your head stands on end, your soul lies in your hand."

For another example from 490 B.C.: Herodotus writing of the battle of Marathon cites an Athenian warrior who went permanently blind when the soldier standing next to him was killed, although the blinded soldier “was wounded in no part of his body.”

You might be interested in some thread from the FAQ section on "Post Tramatic Stress Disorder before the modern era":

• Did Ancient peoples suffer from Post-Traumatic Stress Disorder or similar psychological issues? - 55 comments, over 2 years old.
• There's a wealth of examples drawn up in this thread though not as much analysis.
• Is there any information on Post-traumatic Stress in societies in which warfare was looked at in a more favorable light (e.g Sparta)? - 41 comments, over 2 years old.
• The topmost comment draws examples from the thread above this, but down below there's a source brought in to analyze pre-modern combat which results in a debate both about such occurrences and the value of the source.
• Are there any indications of combat PTSD in societies like the Spartans, Khan's or Alexander's armies? - 31 comments, over 1 year old.
• This thread narrowly focuses on Roman soldiers and thus is able to do strong analysis of the available texts and evidence in a way not done in the threads above.
• How was PTSD (Post-Traumatic Stress Disorder) treated in the Middle Ages of Europe (or anywhere during 400A.D. to 1500A.D.)? How well documented are we upon the subject? - 26 comments, over 10 months old.
• This thread focuses on Early Modern Europe and also deals with the difficulty of retroactively diagnosing people through their writings.
• Is there evidence of PTSD in ancient warrior cultures? - 14 comments, over 1 year old.
• A few more examples ancient examples as well as dealing with a book on the topic of PTSD.

Here are some more threads that I found on the topic:

• How did the soldiers of antiquity/medieval times deal with the inevitable PTSD type issues that come from brutal hand to hand combat? - 147 comments, over 1 year old.
• More examples from all across time, with analysis and source criticism present in several of the comment threads.
• Did soldiers in the middle ages get PTSD? If so, was it different then PTSD now? - 83 comments, over 2 years old.
• There's some debate in this thread whether or not the circumstances that people lived in during the Middle Ages made them more resilient or not to PTSD.
• How common was PTSD in 18th and early 19th century armies? Particularly in Napoleon's armies - 4 comments, over 11 months old.
• One commenter talks about a high level officer who may have experienced PTSD such that it impacted his decision making at a critical moment in a major battle.
• [Is there any mention in medieval books and records from those times of what we now call PTSD in knights, soldiers, warriors etc?](Is there any mention in medieval books and records from those times of what we now call PTSD in knights, soldiers, warriors etc?) - 9 comments, less than 1 month old.
• There's one large quote given in this thread about how clergy specifically cited the psychological toll of combat as a reason against it.
• Was PTSD any more, or any less, prevalent amongst ancient and medieval soldiers than todays soldiers? - 16 comments, over 2 months old.
• This thread delves into a discussion of the merits and flaws of an often cited work on PTSD amongst pre-modern warriors.
• Did men of the past suffer emotional trauma from war and killing? - 6 comments, less than 1 month old.
• This thread goes deep into the source criticism and interpretation when making judgements of whether or not pre-modern peoples were experiencing what would be diagnosed as PTSD in the modern day.

Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, questions of this type can be directed to more appropriate subreddits, such as /r/history or /r/askhistory.