An undead mob on minecraft is one hurt by a splash potion of healing and healed by a splash potion of harming. These are all the ones that are undead
Zombie
Skeleton
Zombie pigman
Zombie villager
Wither Skeleton
Baby Zombie (Unused)
Human (Removed)
Baby Zombie Villager
Many people say the unused mob "Giant" is undead because it is a mob that is 12 times bigger in the zombie, which is undead, but it is actually damaged by a splash potion of harming and healed by a splash potion of healing.
That all depends on what you want to accomplish using the sword. If you want to kill undead mobs easier, then smite. If you want to push back mobs, then knockback.
Smite increases damage to undead mobs."Each level separately adds 2.5 ( × 2 1/2) extra damage to each hit, to "undead" mobs only" - gamepedia
You can download mobs minecraft comes alive
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.
Put it on peaceful
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All hostile mobs spawn on easy difficulty.
If you mean in all mods ever, then potentially thousands. Mods are add-ons to Minecraft created by users, and new ones are made every day. To count how many modded mobs there are in Minecraft would be near impossible
Mobs are enemies, so I suppose that there wouldn't have been.
No, mobs will not spawn in water or ontop of half slabs.
Because it hasn't updated yet. When they make the next version of Minecraft IOS, Then there will be all of the things that are in PC or Mac Minecraft
Creepers can not have kids in vanilla Minecraft. No hostile mobs in Minecraft can be bred.