The House of Rohan was a dynasty of French nobles from Rohan, Brittany. The early branch existed from 12th century to about 15th century or so. My question is: to what extent did medieval Brittany (and, by extension, the House of Rohan) make use of cavalry?
Although detailed information about that period is pretty sketchy, it's clear the Breton armies contained thousands of heavy cavalry. Here's a short survey of battles explicitly mentioning Breton cavalry during that period.
Fought between 1341 and 1364 between the Counts of Blois and the Montforts of Brittany for control of the Duchy of Brittany. There is a clear account of thousands of French cavalry at the Battle of Morlaix (Sept 1342).
[Contemporary English chronicler] Adam de Murimuth puts the total French numbers at 3000 cavalry, 1500 Janissaries by which he may mean Genoese and a mixed force of Breton infantry.
This war saw the climax and fall of heavy cavalry with longbows, firearms and horse-mobile light infantry (they ride horses but dismount to fight) finally gaining the measure of heavy cavalry. The cavalry of Brittany featured in several battles.
At the Battle of Formigny, Breton cavalry showed up to assault the British flank and force a disastrous British withdrawal.
At this time the Breton army under Arthur de Richemont arrived from the south, having crossed the Aure and now approaching the English force from the flank. They numbered almost 1,200 Bretons – almost all mounted judging from the pace of their march.
And again at the Battle of Castillon, Breton cavalry dealt the death blow.
Despite the odds against the English, the battle lasted over an hour until a thousand-strong Breton cavalry led by Peter II, the Duke of Brittany crashed into their right flank, sending them into retreat.
To add to Schwern's answer, and to answer Pieter's remark that europe had no native light cavalry; After the Bretons came to Armorica from Cornwall, they brought with them a tradition of light skirmisher cavalry dating back to the Celts.
At the Battle of Balon in 845 about 1000 Breton cavalry defeated as much as 3000 Frankish infantry. The Frankish forces had been ill prepared, so 5 years later Charles the Bald sent a new army, with Saxon light infantry to deal with the Breton Cavalry. The same number of Bretons faced between 4000 and 6000 Franks. Instead of coming to grasps with the Franks they harassed them for to days with javelins. On the second night Charles fled, leaving his army with no choice but to do the same in the morning.
These cavalrymen would have been equipped with black-painted shields, swords, javelins and sometimes armour. We know of one occasion where a Breton warlord took an extra wagon with a 1000 throwing spears with him, which might explain how they had enough ammunition to keep harassing for two days.
De Bello Gallico
Cassard, J., Les Bretons de Nominoë