Special Zombies are not uncommon but far fewer in number. But when they show up you'll wish they hadn't! The Variants are as follows:

Security Zombie:

These guys are tougher to take down and hit harder than other zombies. They have similar behavior to other zombies but are clearly identified by the buzz cut, security gear, and the size.

The best tactic for this enemy is to just pour into him with a high DPS gun until he stops twitching. Avoid melee.

Grenade Zombie:

Also clearly marked by their big riot helmet, grenade belt, security gear, and being very tall and lanky. They'll lob grenades at you, suicide charge with explosives, and generally be a pain.

Generally speaking what you want to do with this zombie is level a well placed shot to the head - they are relatively weak compared to other specials in this list so they go down quick. If you have room to run and the horde is following you, these guys will throw grenades into the crowd, actually helping you quite a bit if you stay out of the way. You may want to keep him around for 2-3 throws and run around before shooting him if you're low on ammo to help deal with the horde.

Minikin Clone (melee):

Unmistakable, these fat open vest zombies are high on health and hit very hard, but are also very slow. They also make deep grunts and growls so you can hear them coming.

They're easy to deal with so long as you have ammo - if not, carefully make long cuts or stabs to the head with a sword or knife to dispatch them.

Minikin Clone (chainsaw):

Like the Minikin melee clone but with a giant chainsaw and a boatload more health. The Chainsaw is slow to hit and you can hear it farther away than any other zombie sound in the game, but if it connects it will do a lot of damage. Use the same tactics as with the normal minikin clone, but back off once he begins to attack.

Dead Space Zombie:

This Zombie has a gigantic astronaut helmet and gear, broken in the face plate to reveal a bloody skull. This enemy has the highest health of any special zombie in the game and typically does not engage in melee. They're slow and they'll shamble toward you doing one of two attacks: when in range they're let out a high pitched scream which will damage your health and blur vision in the area in a cone expanding from their mouth, or they'll vomit acid on the ground.

Shoot in the head with high DPS guns with medium or long range is the safest strategy, but they can be killed with melee easier than other zombies because they have slow attacks.