Hey, everyone! Since we get plenty threads about changing sims into creatures, I thought it would be a good idea to make this thread. I will tell you how to make a sim into a certain creature, as well of telling you the cheat way and what the sim acts like.
So, lets get started with...Aliens!

How to Get a Alien Sim
You do not need and Expanison packs to get an alien sim. Their is a small chance that if your sim looks through a telescope, they will get abducted by aliens. Once they return, if they are male, they will become pregnant with an alien baby. If you have boolProp testingcheatsEnabled true on you can shift click on the telescope and choose "get Abducted" OR if you have boolProp on, shift click on the sims and choose spawn>the tombstone of L and D> make me alien pregnant.

How to get a zombie
You will need the expansion pack University. Get the career reward Resurrect-o-Nimitron (Paranormal) To make a zombie, you'll need the sim you want to become a zombie dead, and have a sim who knew the person dead. Use the object and pay the Grim Reaper not that much money. I do not know if there are any cheats for it.

How to get a vampire sim
First, you'll need the expansion pack Nightlife. If you have it installed, have one of your Sims go downtown at night. You need to find a vampire. Vampire's wear victorian style clothing and their names start with Count or Countess. Become friends with them, and they will bite you. Be careful, if Vampires go in the sunlight, their needs drop and they will eventually die if you keep them like that. But a coffin for them to sleep during the day. To cure them, purchase a poition from the Gypsy. There is a boolProp cheat, which you can do by shift clicking on the sim and selecting "Make Vampire" on the 2nd or 3rd pie menu.

How to get a servo sim
You will need the Expansion Pack Open For Business to get a servo. Have a sim with high skill in building things build a servo with one of the thing making benches. If they have enough skill then they will build a servo which will do work for them.

How to get a werewolf sim
You'll need the expansion pack Pets. If you have a house with lots of trees, bushes, flowers, and a lot of babies and toddlers, their is a small chance that a black wolf with glowing eyes will show up. Become friends with the wolf, and it will bite you and infect you. Sims will turn into werewolves at night. To cure them, purchase a potion from the animal trainer.

How to get a plant sim
You need the expansion pack Seasons to get a plant sim. If your sim sprays their plants a lot, they will turn into a plant sim. If you buy a potion from the garden club, you can turn human again.

How to get Bigfoot
You need the expansion pack Bon Voyage. Have a family go on a trip to to three lakes. have them dig until they find the map to the secret lot in three lakes. go there, and become friends with bigfoot. after you grow close, you will have the option to ask bigfoot to come home with you. if he accepts, when you return home from vacation, he will be there.

How to get a witch sim
You'll need the expansion pack Apartment Life. Meet a witch on a community lot. You'll probably be able to identify one of them. Bad witches wear black and Good witches wear white. If you want to learn good spells, become friends with a good witch and then ask to teach you the ways of the light. If you want your sims to learn bad spells, become friends with the bad witch and then ask to teach you the ways or the dark. You can give up magic and time you want.