Humans
Though the world is a nuclear wasteland, there are still a few shady characters out there.
Raiders
Raiders are people who kill for profit or fun. They enjoy torturing anybody they capture. They wear self-made armor, usually adorned with spikes. They use a variety of Melee weapons and Small Guns such as the .32 Pistol, Tire Iron, and some may carry Frag Grenades. They also carry Big Guns including the Flamer and Missile Launcher.
Talon Company Mercs
These guys will come after you if your character has Positive Karma, or sent after you by Mr. Burke if you disarm Megaton's bomb. They are known for taking jobs that other mercenaries won't take, and for being very brutal. They mainly use Assault Rifles or Laser Rifles, but some carry Combat Shotguns and Laser Pistols.
Regulators
These guys are the closest thing to cops you'll find in the Wastes. They wear cowboy-style clothes and are basically bounty hunters. Regulators will come after you if you have Negative Karma.
Animals
The fallout from the nukes have caused many creatures to mutate, including animals and insects. Animals are guaranteed to have a piece of meat on them when killed.
Some animals can become friendly towards the player if he has the Animal Friend perk - even aiding him in battle if he has the second rank of the perk.
Radroach
Just a big cockroach. Their attacks don't do much damage, and can be easily killed.
Mole Rats
A mole rat is a large rat.
Dog
Some dogs have mutated in the wasteland and will attack.
Bloatfly
A large fly. They can easily sneak up on you, and can be annoying, especially in groups. But they are slow and weak, and easily killed.
Radscorpion
A large scorpion. They move pretty fast, and should be taken seriously. An annoying tactic solo Radscorpions like to use is running away from you in combat, only to join up with another Radscorpion. Larger versions also exist, with the same general look as a normal Radscorpion, but is much larger, with stronger attacks.
Yao Gui
Yao Gui are radiated black bears. They are very strong in close combat and will very quickly rush you. Usually, they are found alone but can be found in pairs. Shotguns are very effective but if spotted early they can be killed easily with an assault rifle or other long range weapon.
Yao Gui meat is valued, since it provides a 10% damage boost in addition to it's normal HP gain.
Deathclaw
Deathclaws are an enormously large, agile, and strong species of mutant reptiles created through genetic engineering. They are substantially larger than a human, and have large and dangerous claws. They are probably the most difficult monster in the game when not handled correctly. Taking down a Deathclaw requires heavy preparation. They have high health and high damage, and very deadly for several other reasons, including fast movement speed, larger than average pounce range, and sweeping claws that are difficult to avoid. However it is encouraged to use Melee or Unarmed weapons to take one down with a parry-hit-parry-hit tactic, even though an unarmed fighter with a level of 18 or higher can easily take one or two of them without taking a hit with the Paralyzing Palm Perk. At minimum you should use mid to high-powered Small Guns up to take one down, maybe coupled with some explosives for good measure.
Note: Deathclaws do not appear to be affected by the Animal Friend perk.
Mirelurk
The Mirelurks are a species of bipedal, mutant crabs. Most Mirelurks are surrounded by a thick armor/shell which is difficult to penetrate. However, their face is vulnerable and is the best area to attack. Melee Weapons are basically useless unless you use a very powerful weapon like the Sledgehammer or the Shishkebab. It is preferable to keep Mirelurks at a great distance, when possible. The Dart Gun will do decent damage over time, and can help prevent Mirelurks from lunging at you.
- Nukalurks: A little stronger than Mirelurks, but can be taken down using the same tactics. They are found in the Nuka-Cola Factory.
- Mirelurk Hunter: A much stronger version. An initial flame attack that gets used against you, which can disable you for a few seconds while you get attacked (even when you are in V.A.T.S.) Other than dodging the initial firestream, you can follow the same tactics.
- Mirelurk King: A mirelurk with a softer shell and exposed head; unlike regular mirelurks, there is less need to have great accurracy, but they do have a ranged attack.
Note: Mirelurks do not appear to be affected by the Animal Friend perk.
Robots
Robots are mechanical units that are programmed for the company or organization they work for. Sometimes they are friendly, but they can also be hostile.
If you have the robotics expert perk and can sneak up on a robot, you can instantly disable it. If that fails you, you can throw a pulse grenade to disable robots as well.
Protectron
A protectron is a robot with a laser weapon. The first example you will see is Deputy Weld from Megaton.
Robobrain
A robot with a laser weapon. It is identified by its tracked propulsion, and it's head that resembles a human brain. It is normally used for more complex service tasks, as with Vault 112.
Turret
A turret is a fixed emplacement that fires machine gun bullets at hostile targets. Usually, there is a computer terminal that allows changing their targeting system, to either disable it or attack everything.
Mister Gutsy
A military version of the hovering bot that is a variation of the household butlers or utility bots. It normally is equipped with plasma weapons.
Creatures
Some enemies were once humans that mutated, or were created by the government. Some of them are big and dangerous.
Feral Ghoul
Ghouls that have "gone zombie". They often attack in groups, and sprint towards you at high speeds. They can be easily killed with a close-range weapon, such as the Combat Shotgun. There are special highly-irradiated Ghouls called Glowing Ones, which glow green, and use a special attack to release radiation that damages you, and heals all nearby Feral Ghouls.
Super Mutant
Super mutants are mutated humans. They are much taller and bulkier than humans, have (mostly) green, gray, or yellowish skin, are immune to disease and radiation, and are gifted with superhuman strength and endurance. Most Super Mutants either have a Melee weapon, an Assault Rifle, or a Hunting Rifle. If you have a Hunting Rifle, wait until they get to within 20 yards and then enter VATS. Successfully hit their head three times and they will drop, skull intact or not. Since a large majority of Super Mutants have Hunting Rifles and carry six bullets, this tactic is a great way to make money and collect useful ammo. Super Mutants are divided into different classes, each more powerful than the previous. These classifications, in order of weakest to strongest, are Super Mutant, Super Mutant Brute, and Super Mutant Master.
Super Mutant Behemoth
A Super Mutant Behemoth is a giant Super Mutant. There are five in total, and they do not respawn after you kill them. Behemoths are roughly 15 feet tall. They have short, stubby necks and extremely large build. Some Behemoths carry fire hydrants which can easily damage, and swiftly kill any living being in their radius. Never stay still as Behemoths are highly effective close combatants and will do significant damage from Melee range. Moving backwards while firing is particularly effective. Never let yourself get cornered or stuck on terrain unless you want to be turned into a pile of mush. The Fat Man and 3 mini nukes can take down one of these very quickly and is very effective