I noticed today, after being reminded of the episode in The Fellowship of the Ring, when Strider and the hobbits are crossing the Midgewater Marshes, where they hear countless noisy insects,
There were thousands of them, and they squeaked all round, neek-breek, breek-neek...
for which Sam coins the name "neekerbreekers," the name is awfully similar to the name of the Black Dwarf in Prince Caspian: Nikabrik (especially given the non-rhotic English spoken by the two Oxford don authors).
The similarity seems unlikely to be a coincidence, since Tolkien and Lewis were good friends. However, I am wondering whether there is any more information about the similar names, especially who came up with the sound first. Prince Caspian was published in 1951, so after most of The Fellowship of the Ring was written, but before it was published. Given this timeframe, and since Tolkien’s usage has an in-story onomatopoetic origin, while Lewis’s apparently does not (and as this blog post points out, the way Tolkien uses the name is also suggestive of nicker for a water spirit), my initial guess would be that Tolkien coined the name first, but I wonder if there is any additional information either way.