Jumpers???

[ZombiesBeLike]

Yeah exactly

[PhysicsLB]

I liked my old theory that the virus has an evolutionary effect on its host.
When you look at how things played out it kind of makes sense.

With the outbreak, the infection spread quickly and in the panic people were easily chased down and turned/killed.

The runners/jumpers were a response to the light fortifications people used to keep the regular zeds out. They could chase down caravans and jump over fortifications or into buildings.

The behemoths show up after this, and are a response to heavier fortifications. Sheer power to break down walls or get into heavily fortified buildings. They didn't need to be as fast because it was harder for people to get away while inside buildings.

However, I think the little ones are the actual "experiments" due to their combined traits and the tatooing.