I don't know much about them, but probably because history is written by the winners, and the Romans beat the Celts (??)
The main reason is propaganda. The Romans presented their wars as defensive, as justified and they had to obtain casus beli or the Gods won't do them favour. And it turned out that it works if you present your enemy as an animal.
There are many more nations presented as such - huns, germans, vandals, the … Read more