Can animals get shellshock too? Like there were alot of dogs and horses especially in ww1 at the frontlines. After the war or also during it, did animals get "trauma" too? If yes, how did it look like in animals?

I'm a veterinarian rather than a historian, so I'll try to answer this from a medical rather than a historical perspective. The short answer is yes.

It's challenging to make a diagnosis of mental illness in an animal, since we can't ask it questions or obtain a comprehensive psychological profile. Thus, I think any veterinary behaviorist would be reluctant to use the term "shellshock" or "PTSD" to describe an animal. However, we can observe an animal's behavior and extrapolate to make conclusions about its mental state. When a dog or horse is exposed to a traumatic event, especially repeatedly, it will react with fear and anxiety when exposed to the same or a similar "triggering" stimulus. We presume that the animal is remembering its previous trauma and reacting accordingly, which is analogous to human PTSD. In the wild, this would be a protective response--something happens which hurts the animal, and it learns to avoid that thing. However, when the animal is exposed to extreme stress and trauma, the fear/anxiety response can be equally extreme, leading to inappropriate fear.

For example, consider a dog who has a firecracker shot off near its face. After the initial incident, it learns that firecrackers are frightening and painful. If it hears another firecracker, it will run away and hide. Sometimes, though, the dog will begin to generalize that fear. In addition to an appropriate fear of firecrackers, it may develop anxiety when exposed to any loud noise--thunderstorms, slamming doors, revving cars, etc..

I expect something similar would happen to animals in wartime. In addition to the loud noises and strange smells, they will feel the fear and stress of their handlers, which could encourage them to develop a fear response when exposed to any of those stimuli after the war. There is evidence that modern war dogs can develop PTSD-like syndromes. Unfortunately, there is less information on service animals during the First World War. I've found a couple of reports of horses showing extreme fear on the battlefield (stampeding, bolting, etc.), but I have not encountered any information on the long-term behavior of horses that survived the war.

You also ask what "PTSD" would look like in an animal. Generally, it would manifest as fear and anxiety, in response to any stimulus which is reminiscent of its traumatic experience. For example, a dog would pant, act restlessly, whine and yawn excessively, attempt to hide, or display signs of fear aggression if it can't escape the stimulus. I'm less familiar with signs of fear in horses, but they should be similar: shying away, refusing to follow direction, running from the stimulus in extreme fear. In the long-term, an animal might eat less, lose weight, develop gastric ulcers, lose its hair coat, and display other signs of chronic stress.

References:

UCL article discussing horses in WW1 and the stresses they encountered

Short article discussing some individual cases of PTSD-like syndromes in animals

Psychiatric research article which mentions some of the challenges of evaluating or diagnosing stress and anxiety in animals