Once you've gathered all your Obsidian and built yourself a portal to the Nether, you'll find yourself in unfamiliar territory (well, you will if it is the first time you've been there). There are lots of strange creatures down there -- some easier to kill than others. I've written this guide to give you all the information you need to deal with the unique mobs with living in the Nether in Minecraft.
Zombie Pigmen are passive until they are attacked. If you do this, and there are other Zombie Pigmen in the vicinity then prepare to get mobbed (unless you managed to one shot it, in which case, others won't come). Your best bet is just to avoid them.
These fiery inhabitants of the Nether are pretty hard to get close enough to in order to attack them with your sword. A good way to kill them is to use snowballs as they deal three damage on the mob. Watch out for their fireballs though as they deal five damage to players. Blaze's drop Blaze Rods (which can be used to craft End Rods), Blaze Powder, or a Brewing Stand.
One of the best ways to kill a Ghast is to knock back it's fireballs. You can use any tool to do this. Alternatively, you can fire your arrows at its tentacles. Once you can get in melee distance, they are quickly killed by a sword as Ghasts only have ten health.
It's best keep these guys at a distance, so use your bow and arrows. When you get attacked by a Wither Skeleton, you get inflicted with Wither. This effect inflicts one damage every 2 seconds and turns your heart bar black. They are unusually tall, 2.4 blocks tall, so, if you are in a fortress and can quickly build a wall with a door, they are unable to get through. This makes for an easy kill.
These little guys are the Nether's equivalent to slime. The best way to kill them is to use your bow and arrows on the big ones and then go in with your sword to get the smaller ones. Magma Cubes jump around slowly and randomly until you are in range and then jump quickly towards you.
I hope you have found this little guide to the beasts of the Nether useful. Maybe you have different hints or tips you'd like to share? I'd love to hear them!