Tier 2 mobs

These mobs would begin spawning after the world hits an in game year in age, giving you plenty of time to get settled into your world, even then they are not that common and only spawn in certain biomes

just some examples:

A giant scorpion that poisons, It is twice as strong as a spider and moves faster (though it does not climb) it has a chance to drop a scorpion barb which can be crafted with sticks to make a barb tipped spear which has a long reach and has a chance to poison)

Or a Leprechaun that only spawns after it rains, he does no damage and runs away on site but killing him will drop valuable gems, gold, and redstone pieces

Those are just examples, the actual idea is a tiered system of monsters that begin spawning after the world gets older, they can be whole new monsters, or more exciting varients, just a little extra challenge or a rare treat to start finding in late game, they should not spawn with such frequency that they mess up the core game