First thing's first, the mongrel is now (or again) known as the Zombie Dog. It was the name that was used when the idea was first proposed and all the way through development. Calling it a mongrel was kind of confusing because everyone was so used to calling it a zombie dog for so long, no one remembered to call it a mongrel (dune thresher and fallen imp suffer from it as well to a degree). The name zombie dog doesn't need to be lore-fied really. It describes the skill perfectly, so why not use it?
So anyway, right, the fire and poison/locust enhancements for the zombie dogs was removed. It was a cool idea but it just wasn't really panning out to be anything meaningful. It was sort of confusing as to why you would be switching between fire and poison, was it to keep an additional DoT active, or maybe there would be tactical reasons for it? But it just wasn't really jiving in a way that made sense for the rest of the game as it all came together more. It wasn't shaping up to be a meaningful or fun decision to make on-the-fly. And if you didn't happen to spec into the skills that would empower the mongrels in different ways, what then? You're just constantly refreshing a fire DoT on them? To what end? It seemed more and more that a decision that was made at the base skill, either through spending points in another skill (passive potentially), or using particular runes to alter the zombie dogs, made the most sense.
I haven't spoken to the designers about it, so I could be way out of line, but I think the potential for passives that affect all "summoned zombies" is definitely there as there are quite a bit more of them that exist now. The gargantuan, zombie charger, wall of zombies, grasp of the dead, and then the zombie dogs of course.
