Did the Knights of the Middle ages actually use daggers? Research so far is sketchy but we can find evidience of the baselard (also sometimes referred to as Basilard) in use as early as the 13th century. This dagger has a curved pommel (the counterweight at the top of a sword) and cross guard. It was used in the region we now know as Switzerland. But the weapon was most popular with Swiss foot soldiers, not the knights.
There was also the Cinquedea is a civilian long dagger of Italian origin. It’s name means five fingers. That name was developed to refer to the width of the blade at the point that the blade met the crossguard. The cinquedea was used primarily as a thrusting weapon. Because it was carried horizontally next to the buttocks so that it could be drawn laterally from the back, it does not seem to be a weapon that a mounted knight would use.
Find out a lot more about medival weapons by browsing this entire site.