The Centurion and his horse questions.

[Magnus]

I don't think a few steles showing a centurion with a horse is indicative of anything beyond the fact that he simply has a horse. I think Mr. Webster may have been stretching things a bit by coming to the conclusion that they rode on the march. It also seems odd (not that I'm arguing with Arrian) that centurions would be that far away from their troops while on the march...optionnes or not.