Hermann and Hermann (they share the same name) are a pair of black Oriental brothers that appear in the first novel and the movie.
They act as minions and followers for Kong. At first they taunt Francis and Bluebeard, backed by their boss. Later they are directly involved attacking Francis but the cat manage to escape.
Both of them are neutered, and Bluebeard mocks them because of this.
They are also mentioned in the third novel, Cave Canem, still backing Kong.
Trivia: In the movie one of them receive a distinctive gray coloration, probably to differentiate them.