The facial expressions of mice. Researchers have described different emotional facial expressions for mice. Similar to humans, the face of a mouse looks completely different when it tastes something sweet or bitter, or when it becomes anxious. With this new possibility to render the emotions of mice measurable, neurobiologists can now investigate the basic mechanisms of how emotions are generated and processed in the brain. Source 4v.
I mean just like how humans will have emotions and express them through their own body language and facial features; animals do as well! Unfortunately society does not always pay attention to that and focus on other aspects of the animal that may not point out that it has feelings as well.
In his day, the psychologist Paul Ekman determined that there were six common expressions: joy, sadness, fear, disgust, anger and surprise. Well, this research has determined that fear and surprise use the same gestures as anger and disgust.