Verdant Forest really is the weakest forest.
Not only are most of the animals we've seen in it giant versions of either small animals on Earth (birds, snakes, insects and such) but the lord of the Forest is a herbivorous mammal with a quite peaceful reputation (not that actual elks are peaceful at all, they're dangerous as fuck and both agressive and deadly, but in countries that don't have them, they're mostly associated with gentle stuff due to their appearance that is close to a deer).

Now you take the Forest adjacent to the Lawless territory and a (probably) fodder animal in there is a fucking bear. That whole Forest is likely mostly oriented towards carnivorous predators and big animals (turned even bigger). The Lord there might be a fucking tiger or something like this. AND instead of having a zombified bear from the get go, the Parasite had to wait for the bear to die first in order to take over the body, which is already a sign that the predators there are much stronger and harder to infect than those of the verdant Forest.

Every Forest above that, like the Stygian one where the Madaran team disappeared, will have even stronger "families" of beasts with even higher resilience to the Parasite.

tl;dr
>Verdant Forest: Mostly herbivorous or small animals, get zombified instantly by Parasite. Lord: an elk.
>Crimson Forest: Probably 90% of carnivorous beasts, need to die first for the Parasite to take over. Lord: ???
>Stygian Forest: Literally giant dinosaurs? Can digest Parasites as easily as normal food?
>Azure Forest: Abyssal sea-creatures but living on land?