Mobs exist by spawning. Spawning can happen in various ways. They naturally spawn in their spawning environment. They usually spawn in groups with other mobs of the same variety. Players in creative mode can spawn most mobs from spawn eggs from their inventories. Mobs will not spawn on transparent blocks, in water (except for squids and guardians), on half blocks (slabs, stairs). Monster spawners are an exception, as they can spawn on any block, including air. Mobs can also spawn from portals or from other mobs. Mobs like the snow golem can be crafted. Unused mobs like the giant are not used in the game currently, but they can be summoned by the /summon command.