Is there a "canon" of Zen Koans?

There are lots of Koans on the internet, but I am not sure if there is some sort of process that a Koan must go through, or if there is some sort of list of Koans that are taught and that new ones are not added. Meaning, if a Koan is just made up on the spot by some random person, and the Koan is unrelated to the core principals of Zazen, I presume it wouldn't be taught by a Japanese Zen master. Which leads me to believe that there are some sort of "canon" of Koans.