Seeing as to how this is at night in the real world equivalent of the Overworld, I'm assuming this doesn't include Nether creatures, or either of the bosses, as well as ignoring the friendly/neutral mobs that spawn in the daytime. read more
Assumption 1: ALL the mobs of Minecraft are able to spawn at night, excepting passive mobs other than cats. Assumption 2: We are treated by the mobs as player/villager hybrids, which zombies will convert to more zombies and other hostile mobs will attack. read more
Minecraft works in a chunks system, as do all entities in minecraft. Therefore, if you are not in a certain area of those chunks, the mob will despawn. This, however, can be changed in three ways. read more
The other jargon used here is spawn, which is the word for when the computer makes something appear. Hostile Mobs. These are the ones you have to worry about the most! All of them spawn in the dark — either at night-time, or in caves, or in your house if you don’t provide lighting. read more