18907
The Greek military writer Arrian describes these in his book The Art of Military Tactics ( Technē Taktikē ) , and says they originated among the non-Roman cavalry units provided by the allies ( auxilia ) , particularly the Gauls ( that is , the continental Celts ) and Iberians .
 Arrian, Iberians
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Arrian, Iberians