The change may be of different types and that varies with each and every organism. In general, the neuroscientists say that nearly all animals have some sort of neural mechanism of responding to various situations like danger, caution etc. Some animals those who are close relatives of human show almost the same mechanism as humans. Like a chimp or a bonobos shows mostly the same expressions as humans do. But it is never possible to read by every human in the society. It can only be proved by scientific researches.
When we talk about the external physical expression in animals, you may notice some expression in animals as well. But how this can be detected? No it is hard to understand in animals but the animals like dogs and cat shows some common fear expressions. The eyes are the main indicators of fear. It is easiest to understand by looking at the expression of eyes. As the eye structure is different in animals so the expression also varies. Some animals, show their sclera, again some shows the white of their eyes. See we also do almost the same. Our eyes widens in both fear and surprise.
So the fear in animals is a common sign and if we try we can assume it from their physical or facial expression. But if you want to gather a lot of knowledge in this topic, you will have to study a lot about animal psychology. Here you have seen some amazing pictures of animal showing expression that they are in a fear or a horror. So pictures are the greatest examples of all time and we know that very well. For that purpose we have collected the pictures from various sources in the internet to show you the horror in animals.