Why might a Roman historian like Ammianus want to compare the Huns to “unthinking animals”?

Because from roman point of view, one who is not roman were considered barbaric creatures.

Explanation:

The Huns were barbarian nature should be seen in the context that they were very fine horsemen, great fighters, and vicious in battle. They did not fight with the methods of the Roman war, and the Romans did not fight with this barbarity. If an individual was not a Roman, he or she was seen as a barbarian and therefore seen an animal that should be treated harshly.