Whatever happened to the "giant" zombies that were to be introduced?

Versions of the Behemoth exist in some overhaul mods like Darkness Falls. There is another overhaul that utilized the same methods to create a number of other custom bosses, but I cannot recall the name of it currently.

It was shelved because the size of the boss caused issues with pathfinding in the voxel space. I wouldn't put it past TFP to bring something like that back later though for legendary bosses in specific areas with specialized mechanics. Either later in Alpha or as DLC after release.