The writer Wace, was the first one to call Arthur's sword: Excalibur. He tells that Arthur got Excalibur from the Lady of the Lake. It is most likely that Excalibur was made up.

However, the historical person behind Arthur must have had a sword. In the dark ages, a king was recognized by his sword. It could be that Arthur had such a beautiful sword that there were people who wrote about it. At the end of the original story Arthur throws Excalibur back into the lake. This is based on Celtic tradition. When a king died, the sword would be thrown into the lake as a sacrifice to the water goddess1.