This may well have already been explored, but it keeps bouncing around in my head, and I couldn’t find a similar thread!
So, speciation occurs at different speeds in re-world evolution, so this accelerated evolution in the PP universe should have some regions of hyper-accelerated mutation, where the environment is so harsh as to force the Yuggoth fungus to sprint to keep up, creating powerful if short-lived/fatally flawed life forms.
As these species are so flawed, they would not live long, so the only way our valiant heroes would ever encounter them would be if they were forced in to an area shortly after the infection had taken hold, but before the harsh environment is able to destroy all the turned humans. In the event of any creatures surviving, they would likely be far more powerful than usual, and more varied in appearance. It would also allow for different tactical approaches, such as subduing or confounding devastatingly powerful creatures before they die of their own accord, or using fatal flaw in their design to defeat them, with precision aiming, or environmental conditions.
In the one of the early lore eBooks, we looked at the discovery of an ancient civilisation, and saw some of the first human mutations emerge from a pool. One of them was described as looking maladapted, and like he wouldn’t live long. So there is precedent!
I think this could help with the “Oh God, what is THAT thing?!” factor even more - plus allow for some creative ideas that perhaps wouldn’t fit in the game otherwise.
Although we’ve not seen them yet, I must assume that there is a healthy amount of concept art of screeching horrors composed of multiple bodies, used haphazardly by an alien intelligence unfamiliar with human anatomy. A behemoth of screeching corpses, twitching and shambling, its very steps crushing the skulls of its component bodies, reducing its tactical awareness to that of a creature of bestial fear leaving a snail trail of gore across the battlefield? That would work great here!
Maybe a crawling mess of infected viscera, enveloping the corpses of fallen crabmen, snapping their bones and fashioning limbs like a Katamari from hell? I’ll taken seven!