Races in the Fallout world are a bit different then our own, including mutants, wild animals, and robots. I will go over the different types of creatures you will encounter and what to expect, but remember each creature is an individual and you should react to it as such.
Humans: Humans are the most common and most diverse type of creature you will encounter.
Some humans are more or less skilled in combat than others, but you can get a general idea of their
strength by their appearance. Are they wearing a leather jacket and carrying a knife? Or are they
clad in power armor and carrying a gatling laser?
Rats: The first and weakest enemies you will encounter, rats can wear down your hit points,
but are easy to kill and more of an annoyance than anything. Once good critical hit will instantly
kill a rat.
Radscorpions: These are quite dangerous early on, particularly because of their poisonous attack. They can only attack close range though, so just run away if you have problems. If you give their tails to Razlo he will make antidotes.
Mantis: Very dangerous to an inexperienced adventurer. These things attack in swarms and if a pack of them catches you alone in the desert you are in trouble. If you have an attack that affects groups of enemies, use it.
Dogs: You will rarely (if ever) fight dogs, but if you find yourself facing one use a burst attack of some type once they begin to attack you. They can do a lot of damage if you let them, but they are unarmored and will die quickly.
Brahmin: These are the two-headed cows that populate the fallout world. Most characters will never fight one, and if you find yourself facing one, don't worry they aren't very strong
Ghouls: These were once humans, but they were mutated by radiation. They live mainly in
Necropolis and you will know them when you see them, they look like zombies. Most are fairly weak
and armed only with their fists or melee weapons, but they do have a tendency to attack in large
numbers. Burst attacks are highly effective against ghouls but watch your ammo.
Deathclaw: The Fallout Bible describes a Deathclaw as a "mini-tarrasque" These things are
nightmares; fast, strong, armored, and with a large reservoir of hit points. Go for aimed shots or
try flame or plasma attacks. Nothing works great against the Deathclaw so keep your hit points up
and keep attacking him.
Robots: Robots are from before the war and usually guard military installations. Pulse grenades
will only work against robots and do obscene amounts of damage so use them if you have them. You
can attack and kill robots that do not have a power supply while they are helpless.
Floaters: This is one of the Master's "pets"; you will only run into these outside the Master's vault or in random encounters. They have a fairly powerful short-range attack but you should be able to dispatch them easily by this point.
Centaurs: This is also one of the Master's "pets"; you will only run into these outside the Master's vault or in random encounters. These are similar to Floaters, but bigger and slightly stronger. They will still die after a couple of shots though
Super Mutants: These are the toughest strongest creatures
in the game. They carry Big Guns, large Energy Weapons, or attack you bare handed. Whatever they
use, they should be considered very dangerous. Attack them with everything you have or try to
avoid combat all together.