Aside from other players there ere are 6 types of enemies you will encounter on the World Map. Each of these enemies will drop resources or other useful items when you defeat them, they range from Level 1-15.
Plague Zombies
These zombies drop food upon defeat and are the weakest of the zombie types.
Shuffling Zombies
These zombies drop fuel upon defeat.
Ancient Zombies
These zombies drop wood upon defeat.
Mutated Zombies
These zombies drop metal upon defeat.
Rebel Camps
Drop Money
Wendell the Clown
Wendell the Clown is a boss type zombie enemy that appears in random locations around the World Map. Upon defeat he will drop various good rewards which makes tackling him worthwhile. Although Wendell the Clown appears multiple times you will only be able to make one kill a day.
Wendell the Clown appear at various levels of difficulty, the higher the level the better the rewards you will get when you defeat him. You can attack any level of Wendell the Clown but you only get the rewards if your rally makes the final killing blow against him. Defeating Wendell the Clown above Level 1 will take well equipped formations.