You can't, they are only spawned naturally. You can, however, use a spawn egg in creative mode to spawn them.
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By either going into creative mode and getting a Spawn Egg, or by activating "Cheats" in Survival mode and using the command "/give (Minecraft Name) (Number) (ID Number for Spawn Egg/Name of Spawn Egg)"
To find sheep in Minecraft 1.0, you will have to go to a grassy area and look in that area because peaceful mobs only spawn on grass.<(0_0<)(>0_0)>
To find pink sheep in Minecraft, you will need to go to a grassy area to look because peaceful mobs spawn on grass and I have found a pink sheep which is 1/100 likely to find pink sheep but Minecraft wiki doesn't tell you about pink sheep.<(0_0<)(>0_0)>
In Minecraft Classic, mobs can't spawn, so they can't exist naturally. If you play the Survival Test, and when mobs spawn there, you can save it and open it in creative mode so then the mobs can stay there. However, in Minecraft Creative Mode, mobs can spawn, but they can't hurt you because you have no healthbar.
In creative there is a spawn egg for spawning a villager, but in survival you have to find a village, there its a lot of villagers walking around:)!