Zombie animals. It may hurt for some of you, but man, they are all zombies. Primary, mammals, so it makes more sense. I have some zombie animals and variants of them that would be cool ingame.
Zombie bat:
Pretty obvious what it do, it flies to kill you. It is very, very weak, stronger guns kill it with a single shot. It has the ability to "stick" to you ocasionally and keep sucking your blood, damaging you, the only way to remove is if a friend kills it, or maybe if you have a dog or automatic turret. There should be another way if you are singleplayer to remove them from you. Zombie bats would have an interesting spawn, because they would spawn inside building at the CEILING, not the ground, and would stay hooked there until they see a player. Zombie bats would counter people on top of buildings and would drastically nerf that tall building, added in 0.8.0(as there would be a lot of them inside there).
Variant: Poison zombie bat.
Poison zombie bat would be a zombie bat with glowing bulbs on its body, it would blow poison liquid everywhere when killed, which have a very small chance of poisoning. It could also ocasionally spit poison at the player.

Zombie dog:
It would be a very common zombie type, would be fast, and would cause a decent amount of damage, plus good chance of bleeding. Although it would be very weak on health. They would have the ability to howl, calling any nearby zombies and revealing your location. Stealth wouldn't work with it as it can smell you, but it would work if you were full of ( @'Raesk'#2726 suggested gasoline) gasoline/oil on you, as it would camouflage the smell(you still need to be quiet tough).
Variant: Poison zombie dog.
Basically a zombie dog that can cause poison on you 30% of the times it attacks you, and blow up poison liquid everywhere when killed. It would be slightly weaker than a regular zombie dog. If you run a poison zombie dog with a car, the poison liquid would spread on the car, causing damage to the passagers.

Zombie bear:
A rare zombie, would spawn around forested areas, and it would be very strong in health, with medium speed. Its attacks would be slightly stronger than zombie dog's bite. It could also smell you, but with a smaller range. If you ran over it with a car, it would stop the car(like a wall) and damage it, but still the zombie bear would get a lot of damage.
Zombie bear may only spawn on maps that are located in temperate regions, like Europe and US.