Your predators are octopus-like mollusks adapted for hunting plops and whatever else they can get...or they're just bigger plops
First and foremost I have to explain something: no matter what the animal that regularly eats plops is, it necessarily must not be hindered by the rubberizing spell, either by being completely immune to it like some animals are immune to the poison/venom they prey produces (or whatever the honeybadger does to prevent its death when bit by the snakes it eats) or by being adapted to be either unfazed by or even benefitted from its effects, otherwise it'd be unable to hunt other animals, die and result in a situation in which plops still don't really have an actual natural predator either because everytime something eats one it dies or because after enough animals did eat them the rest of the species learned that eating a plop will kill you and started avoiding them altogether.
With that said let's break down the potential drawbacks of the rubberizing spell. As you yourself said:
Basically, when someone kills a Plop with an item, that item is Rubberized; tough, flexible, and waterproof, akin to natural rubber. A Rubberized rock will be firm but springy, like a tennis ball, and it'll bounce. A Rubberized sword will be just as sharp but will be less rigid and more flexible. If someone pours acid on a Plop, the acid will transform into a rubbery substance; if someone burns a Plop, the result will be rubbery ashes; and if one shoots a Plop, the arrow and not the bow will be Rubberized.
However, if someone squashes a Plop into oblivion with their bare hands, or eats one, they will be Rubberized. Do that once, and you'll just affect your hands or stomach (the inside, not the outside). However, those that consume twelve Plops change more dramatically, essentially becoming living ragdolls with unnatural elasticity, flexibility, and tough (and rubbery) skin.
What we can take from this is that if the animal eats enough plops, like an animal evolved to regularly eat them would, it will gain stronger muscles an tougher skin while becoming incredibly flexible. The thing you didn't really specify is whether this would also apply to the skeletal structures in the same degree as every other part of the body. Since you said that it would affect the quills of the hedgehog and make them very soft however, I'll assume here that exoskeletons and endoskeletons alike would simply become something slightly less flexible, but nonetheless very rubbery.
With those assumptions made, I'd say anything that uses a skeletal system to anchor its muscles is out of the game. Why? Because the way our movement works is mostly by the association between the muscles and the bones to move (many of our joints work in a lever like way), something that doesn't really work as well if the bones have turned into a gelatine only slightly less bendy. What we want here in that case is something that already goes by without needing to rely on a skeleton to anchor its muscles to. What could we use then? A magical octopus-like animal.
Octopuses, being a group of mollusks, don't really have a skeletal system, with their muscles being, putting it very simply, anchored to one another. This means that if they were to be rubberized, their overall modus operandi wouldn't change all that much, since they're already known to be flexible and somewhat elastic, with the main difference here being a skin that's tougher than the one they already have. Their bodies would essentially face mostly improvements, gaining a tougher skin that's more resistant to damage caused by sharp teeth, stronger arms that, being mostly composed of muscles tightly packed, could potentially stretch further than they already do, granting more range and an ability to capture prey further away, while suffering no downs, as the rubbery muscles work almost exactly as they did before. So if we make an octopus variant with a more efficient circulatory and respiratory systems, it could easily be a good candidate for official plop predator (as well as potential Apex predator that can essentially subdue and eat whatever its tentacles can get a hold of/whatever it can ambush)
There is however one main down: teeth and hard bits. An octopus immediately post rubberization would have its beak and radula made soft and elastic, aka the only real hard part of their bodies, meant specifically for feeding, will have suddenly become mostly useless for their intended purpose. To solve this problem, I see only 2 real solutions:
1-become more like a snake: these plop eating octopuses have adapted to not really require a beak or other sharp bits to feed, developing larger and longer digestive cavities(and mantle) that make them seem in a way similar to a snake, but with a distorted head full of tentacles. Whenever they find suitable prey, be it a mouse, a bird or a plop they either use its tentacles to throw it in their mouths or simply jump on top of them and chug it all inside, swallowing whatever it catches whole and leaving the digestive track itself to do the breaking down job. This means that the octopus would also work partially like a snake, requiring a good amount of time until it can/needs to eat again. They'd also need to make use of powerful gullet like structure in their digestive track, much like certain raptors, to help crush already dead animals (which were already killed either by the tentacles squeezing it to death or because the neck was broken in the process- all to prevent desperate prey to struggle and make digestion harder than needs to be) or squeeze a plop dead before it can make use of any digestive acid (assuming the acid and digestive enzymes only turn into a rubbery soup if they react with a live plop, because otherwise if they aren't made of something that can be promptly absorbed by the digestive system you can't really have a predator That hunts it to survive, because it'll just go through their system and provide zero nutrition). This complete lack of hard bits also means these land octopuses can essentially act as fluids with almost no limits, since unlike their real counterparts they don't need to worry about the crevice being larger than their beak and the other kinda hard parts have become completely rubbery and extra malleable.
2-"just change em lol": this depends on one critical factor: that the rubberizing enchant doesn't alter every single thing produced by a body that's been rubberized, because if it does, we once again reach the problem related to the assumption in option 1: if everything the body produces is rubberized, that means that the animal can't really digest anything because it's digestive enzymes and stomach acid have been rendered completely useless. With that said, if not everything that the body produces is rubberized, we can solve this problem by simply having the beak and radula constantly replace its now soft bits with new, still hardened ones that, having been produced post rubberization, haven't been affected by the spell yet, meaning they can still allow the animal to bite and rasp at its prey until the ruberizing spell starts affecting them, at which point they'll be again replaced by a new hardened set.
Now, with the pros and cons defined for our octopus monster, why an octopus-snake freak and not just a slug? Firstly because I think the idea of a land octopus with the potential to become an Apex predator thanks to your Enchantment is much cooler, secondly because a slug already exists in your world, and it's the plops themselves. If plops are already an abundant food source in their environment, it wouldn't come as a surprise if plops were to engage in cannibalism, and the fact that we already have documentation that several animals, from several arthropod species to amphibians and even apes, either commonly do or have already engaged in cannibalistic behavior , it wouldn't be a stretch if the plops, not being picky eaters, were to regularly cannibalize smaller plops they manage to capture. If plops Truly are an abundant foodsource, it wouldn't be that surprising if a subspecies of plop evolved specifically to exploit this abundance of food, plus the existence of carnivorous gastropods should be enough to show its not impossible
Summing up, what animals could serve as proper predators for your plops? Option 1, the less interesting: other larger plops, simply because if that smaller member of your species is something you can easily eat, you might as well help yourself and eliminate the competition while scoring a snack. Option 2, the cooler one in my opinion: an evolved species of octopus, descending from a species that lived in a region filled with natural pools near the sea (already Adapted for surviving small periods of time out of water) where plops were abundant and which evolved so that they could more efficiently make use of such an abundant and relatively easy to eat food source, adapting to better make use of their new rubberized bodies and migrating to the interior as a new breed of monster, more adapted for life on land and capable of subduing plops and other animals alike. Whether they can also grow to sizes like seen with the Giant pacific octopus, much smaller or even bigger is up to you. Sure it's not as plausible that such a thing would evolve unless under very specific conditions by earth standards, but honestly neither are slime creatures that spawn out of nowhere and enchant whatever kills them, so I must admit I took a few liberties.
I'm just saying that "you can't go into the forest Timmy, not only you might encounter plops, but you might find the shape-shifting camouflaging tentacled aberrations that eat them, and if if gets you, you're done for" sounds a lot more fitting for a potential Apex predator that can easily eat the things That will gladly eat children by melting them (and let's face it, your plops can essentially eat almost any organic thing smaller than themselves that they can capture, if no other animal eats them, then they're the Apex predators) than "you can't go into the forest Timmy, not only you might encounter plops, but you might find a slightly bigger plop that also eats the smaller ones, and if it gets you, you're done for"
