Question

Why might a Roman historian like ammianus want to compare the huns to unthinking animals

Answer 1

The reason for this is that Romans were at the time a very highly developed civilization while the Huns, together with other tribes with which the Romans have dealt with in previous decades, were mostly nomads who did not have permament and grand settlements such as Rome at the time for example. For that reason, this might have made historians like Ammianus be inclined to think that Huns were savage brutes without the capability to think. 

Answer 2

Roman historians often talked about other cultures and about the impact that these had on the Roman civilization. This is the reason why we often have information about the events that occurred during this time period. However, the way in which they talked about these people was not always positive. Other cultures were often described as barbarians or as animals.

There are two main reasons for this.The first one is that the Romans were convinced of the superiority of their culture, and the power of the empire. This lead them to think of every other group as significantly inferior to them. The other reason is that this was often a political strategy in order to delegitimize the enemy.