Blocks:

Here are the blocks that don't drop from mobs but are found in the timeline. Lavender Oak, Logs, and Bricks.
Lavender Oaks function as trees that are decaying and you can craft logs and cool bricks. You can also craft Crafting Tables as well.

Mobs (Aggressive): 

Ghost Creeper has 100 health and explodes any time it touches the ground. He explodes when killed and drops a loot chest containing Ghostly Gunpowder used for crafting. You can craft the Ghost set giving you night vision so you can see.
Lost Spider will never been seen. All you can see is his eyes. He has 16 health. 
Necromancy Villagers have 30 Health and spawn Medieval Zombies with speed boost. Necromancy Villagers jump high up and resurrect Medieval Zombies to fight for him. When killed, he spawns a loot chest containing a spell book using to craft many things as seen below.  
Super Brainz is super buff with his zombie sword. He does 10 damage and can jump super high. He drops a loot chest containing a Dark Bottle-o. It can be used to craft new weapons.
Little known easter egg that he's inspired from Super Brainz in PVZGW2
LumberZOmb is an agressive zombie with an axe. If he jumps he'll get a insane strength and speed boost doing tons of damage. He can't be escaped and he is faster than the player as well. He has a rare chance droping Dark Bottle-o's.
Xp Demon Guardian has 20 Health and his only purpose is to hide the Xp Lord. If he is striked by the Lightning Staff he'll turn into a Witch spawning the Xp Lord's structure above him containing the boss.
Xp Lord Mini Boss is the boss found on the high structure found from the Xp Guardian Villager when he is hit by Lightning using the staff. When killed he spawns a loot chest containing the XP Bottle of Infinity.
The Xp Block of Infinity is crafted using his drop. This block spawns infinite XP for the player
Herobrine 1500CE is the final boss of the Dark Ages. He has 240 Health and spawns fire and Endermen. Once killed he drops a loot chest containing Year 802.701 Activator that isn't finished and a Herobrine's Infected Block. He also drops a egg containing a peaceful Pet Herobrine.
Pet Herobrine comes from the red egg. The red egg can be taken home just like the way you take back an EnderDragon Egg by either using pistons or torches. In order to get a Pet Herobrine you must place the block as many times as it takes until he spawns by rare 5% chance. This process is annoying but possible.
You can breed 2 Pet Herobrine's useing Dark Bottle-o's. To get a cute baby Herobrine.

Mobs: Passive

Medeval Cows can run very fast. They dropped cooked steak when killed. They can also be breeded with Dark Bottle-o's to create a cute baby Cow.
Medeval Chicken's are black Chickens that drop golden apples when killed. They can also be bred to make a Baby Ender Chicken using Dark Bottle-o
Torch slimes are helpful slimes that spawn torches each area they land. They are really helpful for brightening up the area.
Friendly Helpful Pig is a Pig pet for the player that attacks all mobs in the Dark Ages by range. He has 26 health so he can still be a bit delicate.