The ender dragon can be respawned with four end crystals.

The iron golem can spawn naturally and can also be constructed. Some mobs (like the snow golem and the wither) require that the player 'construct' them before being able to spawn. The exception is monster spawners, from which monsters can spawn naturally on any block including air.

Most mobs never spawn on transparent blocks, in water (except fish, dolphins, turtles, and other aquatic creatures), in lava (except for striders), on bedrock, or on blocks less than a full block tall (such as slabs placed on the bottom half). Villagers also breed randomly depending on the time of day and the number of beds (though they can’t be directly bred by the player). Some mobs including passive and neutral animals, and even hoglins have the ability to be bred by the player, creating offspring. Animals usually spawn upon chunk generation, while hostile monsters spawn and despawn in certain radii around the player. For example, most animals are found in bright areas on the surface, while hostile monsters are commonly found in the dark (whether it's a cave, dungeon, mansion, or at night). Most mobs spawn naturally, depending on the light level, biome, and their surroundings. From left to right: zombie, spider, enderman, creeper, skeleton, drowned, witch, and slime. Some of the many monsters that can spawn in the Overworld. Different mobs spawn in diverse habitats/structures, depending on the light level, biome and surface material.

Minecraft maps future city 1.12.2