well, you have zombie and angry zombie, if you just gave them different skills or attack power you have 2 different zombie types right there. You could create other ones like an agile zombie that can move farther but hits for less damage, or a goliath zombie that moves really slow but hits like a truck. Or you could have zombies that did different abilities, like a zombie that could use ranged weapons or something. Or you could a large zombie that split itself up into several smaller zombies when you got close, and it could guard a really powerful item or something. These are only suggestions; take them as such.