In this scene of Game of Thrones, episode 4x3, a guy on foot duels a guy on horse with lance and easily takes him out by throwing a dagger in his horse's face as he gets close.
Was this technique used much in the past? If every foot soldier had a simple dagger or two to throw at charging cavalry, shouldn't that have made cavalry useless? Was the technique of dagger throwing simply too hard to master, or was the target too small and fast? Or was there a taboo against targeting horses?
Game of Thrones is a television series. It's not historically accurate. This /r/AskHistorians thread (by /u/Bedeage) covers the basics of throwing knives pretty well already. Everything that is said here applies to your question.
Um, I do not know if you have ever been around horses, but they are bigger than they seem on screen. And you would have to be one level-headed marksmen to accurately hit a rider on a charging horse in the midst of a battle.
Then you might say the horse is a larger target, but you have to wonder if a dagger will kill a horse (I doubt it without a lot of luck), and if so, if it will be quick enough to save you.
And there are hundreds of other reasons but someone already posted a link to some so I won't bother.
Honestly, that scene for me is about as unrealistic as when that rhino gets taken down by a single spear in 300.