Mob Bestiary!

There should be a bestiary that you can access using V that shows all the mobs in a book. Whenever you start a new world, you get a new bestiary that has no mobs, and when you kill a mob, it is added to the bestiary, kind of like the bestiary in Terraria. You should also get an achievement from completing the bestiary. Also, you should get XP for every mob unlocked in the bestiary, with the amount of XP varying by the size and rarity of the mob.