Nilson Groundthumper and Hermy are characters created by Stan Sakai who predate Usagi Yojimbo. Miyamoto Usagi was originally conceived as a supporting character in the Nilson Groundthumper and Hermy series, a tale set in medieval times. The Nilson Groundthumper story was envisioned as a 2500 page epic telling the story of the decline of the anthropomorphic characters and the rise of the humans, as well as explaining why there were "intelligent" animals and "regular" animals. Eventually (around page 1000), this story would have shifted from its European setting to Japan, where we would meet the human Lord Hikiji as well as a samurai rabbit named Usagi. But Sakai ended up falling in love with the Usagi character and the Nilson Groundthumper story fell to the wayside.
Sakai has tentative plans to write and illustrate a crossover story in which Usagi meets Nilson Groundthumper and Hermy, but nothing has been officially announced yet.