Nagana, Queen of Evil: 1941, Fantastic Comics#22 (Fox). Originally living 3,000 years ago, she rises in the modern day. She seems to have some magical powers such as calling up flames or her servant Hassan. Or they might be illusions. She is opposed by Kalkor, a high priest of Isis and returning to earth as John Kerry.
Nazi Wolf-Dogs: 1943, Cat-man Comics #22 (Holyoke). In one of the strangest plots ever, Hitler has parachuted into Hollywood dozens of vicious and trained to kill wolf-dogs. With a recording of a wolf's howl, the Hood manages to lead them away from the crowds with a truck and over a cliff. Those that don't die that way are gunned down by the army.
Negus: 1940, Fantastic Comics #3 (Fox). Negus is a powerful witch doctor with real black magic powers. He has a crystal ball with which he can spy on others and makes his home in a great tree. Captain Kidd destroys the tree and possibly Negus himself.
Nekroff: 1941, Big Three #2 (Fox). He's billed as a mad general and he does seem to have an army at his command. He's apparently killed by an explosion in his bid for conquest caused when Samson throws his amphibious tank into a river followed by a tank of Nekroff's poisonous gas which is highly explosive when contained.
Neptune and the Mermen: 1940, Weird Comics #2 (Fox). Neptune is the king of the mermen, men described as having red seaweed beards and red-scaled fish tails (the pics have them having red saucer-like eyes but their demonic faces and beards are yellow, caucasian skinned torsos and red fish tails instead of legs). Despite being mer-men and living on the bottom of the ocean, they don't live in the water. Instead they live behind an invisible wall barrier under intense heat and can breathe fire themselves but have guns that can encase their foes in blocks of ice. They seemingly drown when Typhon blasts a hole in their invisible wall with his ray-gun.
Nicoli: 1939, Mystery Men Comics #4 (Fox). Criminal mastermind who approaches it with a scientific bent. To aid in these endeavors, he's created a gun that temporarily paralyzes its targets as well as a larger machine that will do the job permanently and belts that make him and his gang immune to the effects. He's captured by the Green Mask who uses the gun in later cases (though reduced from a rifle to a handgun). You can tell Nicoli is an evil genius as he has a monocle over his right eye, a hawkish nose, a widow's peak hairstyle and long waxed mustache.
The Night: 1947, X-Venture #1 (Victory Magazine). This masked criminal mastermind, planned some bank robberies and pinned them on Dorne, the bank manager. However, the reporters Rick Rawson and Laura Brown exposed the Night as the bank manager's private secretary, Miss Davis. Even under her mask, she wears a wig for disguise and she's not above kidnapping and murder to achieve her goals. The text of the story describes the Night as wearing black, but the suit is colored green throughout.
Nipon: 1940, Fantastic Comics #13 (Fox). Nipon is cruel dictator of the Orient, with spies, armes, and a fleet at his disposal. He plans on invading Misco/Mexito (Misco may be the name of a city in "Mexito" a country on the South American coast). He also has use of an electronic "voltage net" which is strong enough to capture Samson. However, with the help of his sidekick David, Samson destroys Nipon's forces and captures Nipon, leaving him to the justice of the South American government.
Noman and the Little Men: 1940, Silver Streak Comics #2 (Lev Gleason). The Little Men are dwarfish men with large heads and Noman is their "creation", a large bald man with above average strength, possibly a zombie or android of some sorts. Their goal is to rid the world of beauty and kidnap beautiful women to do so. Mr. Midnight investigates and tracks them down. During a big fight with Noman, a gas that is harmless against humans is accidentally released that disintegrates the Little Men! With their deaths, the women are set free but the secrets of the Little Men went with them.
The Noose: 1942, The Eagle #4 (Fox). The Noose is a villain who likes to kill by strangulation with a hangman's noose. He escaped from jail and set out to kill those who were responsible for his years of imprisonment. When he's stopped from killing the Judge by the Spider Queen, he sets his sights on her. The next day a young woman is found swinging from a noose, with a label identifying her as the Spider Queen. Only he erred, and had killed someone else in her apartment building, which means he got pretty close to tracking her down. Which doesn't phase her too much in tracking the Noose down to make him pay, nor does he seem too surprised to see her alive. It's a battle royal, his rope noose vs. her spider webs. He still almost kills her and it's actually the detective Mike O'Bell that saves her and snaps the cuffs on the Noose.
Norgo: 1940, Big Shot #8 (Columbia). A scientist who developed a method of generating intense cold and used it to freeze various cities and then blackmail the US government. Skyman manages to counteract the cold and then tracked Norgo himself down and sent him to prison.
Norvo: Cat-man Comics #6? (Holyoke). Norvo is an expert at mass hypnotism and a ruthless criminal to the point that he's institutionalized in an insane asylum. His hypnotism is along the lines of many magician characters like Mandrake. Regardless, he's captured by Lance Rand.
Nubo: 1945, Exciting Comics #44 (Better). Nubo is a giant black man in medieval styled armor whose strength puts him on par with the Scarab and a champion for a tribe of Egyptians. When their leader Rajad is kidnapped and a fort of white men are blamed, the Scarab has to rescue Rajad and prevent wholesale slaughter by Nubo.
No. 13: Sparkling Stars (Holyoke). Foreign agent who seems to have decided to go independent as he brings in a gang to raid Lana's brother Rand Reed's camp for his germ-warfare secret discoveries. He then hopes to use the secrets and his international gang to form a Super-Gang Empire to rule the world. His plan is complicated by Fangs, the wolf boy as well as the cruel Maata, the Leopard Girl. In a rare team-up, Fangs recruits the aid of the pilots calling themselves the Hell's Angels to help him capture No. 13 and his gang in issue #30, which was quite a trick as their adventures at the time have them on a completely different continent.