Here is a list of the most common mobs you will find in the overworld and what they drop when defeated
We are going to start of with passive mobs
Chicken
Chickens can give you unlimited eggs but when defeated it might drop 0 to 2 feathers, 1 raw chicken and 1 to 3 experience points.
Pig
Pigs can drop 1 to 3 raw porkchop or 1 to 3 experience points it can also drop a saddle if you put one on it.
Sheep
Sheep can drop 1 wool block,1 to 2 raw mutton and 1 to 3 experience points
Now here are some neutral mobs that only attack you if you attack them first
Wolf
Wolfs can only 1 to 3 experience points
Polar bear
Polar bears can drop 0 to 2 cod,0 to 2 and 1 to 3 experience points.
Finaly here are some hostile mobs that when they see you they immediately come for you.
Zombie
Zombies can drop 0 to 2 rotten flesh 0 to 1 zombie head and 5 to 12 experience points.They also have some rare drops such as a iron ingot,a carrot,a potato,a iron sword and a gold sword
Creeper
Creepers can drop 0 to 2 gun powder 0 to 1 music discs 0 to 1 creeper head and 5 experience points
Skeleton
Skeletons can drop 0 to 2 bones,0 to 2 arrows,0 to 1 skeleton head and 5 to 8 experience points. They also have rare drops such as a iron sword and a gold sword