Hildesheim Cathedral, but I wasn't sure if there might be an earlier example.
After doing a little research, I found a really charming article from 1918 that discusses the history of doorknockers. I was surprised to learn that the doorknocker has existed since ancient Greece.1 At this time slaves were often assigned to answer doors, and they were chained to the door in order to prevent them from running away. The predecessor of doorknockers were short iron bars that attached to these chains, which were used as "rappers."
It appears that the lion's head design also existed for doorknockers in ancient Greece. In 1942 Sterling Dow mentioned some "heavy handsome lion's-head door knockers...which escaped the sack by Philip in 348 BC."2
Lion Gate at Mycenae, c. 1250 BCE). In each case, these intimidating lions serve as guardian beasts for the city, as well as symbolize strength and power. I think the same thing can be said for lion's head doorknockers, which rest on the doors (i.e. gates) as guardians of a building.
On a side note, though, it's interesting that not everyone today associates lion's head doorknockers with such ancient symbolism. This fascinating study by Zachary McCune mentions a woman who selected a lion's head doorknocker for her home, but only because the same knocker was found on the door of the UK Prime Minister's house. In this woman's case, it appears that she wanted her knocker (and her home) to have some connection and/or status with this association to the Prime Minister.
Do lion's head doorknockers have any particular meaning or symbolism for you? Can you think of an ornate doorknocker (of a lion's head or otherwise) that you particularly like?
1 "The Evolution of the Door-Knocker," The Art World 3, no. 5 (1918): v, vii-viii.
2 Sterling Dow, "Review: Excavations at Olynthus," The American Historical Review 47, no. 4 (July 1942): 824.