I was watching the Clive Owen King Arthur movie (which I recognize is not exactly a bastion of historical veracity) and in several of the battles he and/or some of the other knights will, after their initial charge, rear up their horses and dismount to enter the fray on foot. I realize that this is done for the sake of choreography, but it did make me wonder. Were there combat circumstances under which a knight would intentionally dismount and continue to fight, or even enter the battle on foot? If a knight were knocked or thrown from his horse but still able to fight, how might he proceed?
The Battle of Poitiers was a good example, the French knights dismounted and marched to engage the English en masse on foot. They still didn't win, though, but that was due to other reasons.
Being knocked from a horse could cause injuries to a knight, but the main worry would have been being an easy target while on the ground before he could get up, probably separated from his allies and likely with the wind knocked out of him as well. A quick opponent could easily dash up and get a solid hit in, especially if he's isolated.