The Bretons were renowned for their javelin-armed light cavalry and tactics of harassing and withdrawing. On a number of occasions they inflicted heavy defeats upon the Frankish kings.
At some point, probably in the 10th century, there was a changeover towards the Norman way of fighting. By 1066 they probably were identical to the Normans into whose military system they were pretty much fully integrated.
I recently did a unit of Breton cavalry from the plastic set -
http://www.greatescapegames.co.uk/clash-of-empires/stu-blog/breton