In this case, Nymphs means not immature insects but beings born of the life force of nature itself. When magic entered this world from an alternate universe, that magical essence combined with the built-up residual life force all over the Earth. (This assumes A) that every living thing has a soul and B) that when that soul leaves the earth, a remnant or imprint of that soul is left behind, which is the aforementioned "residual life force.") The result was Nymphs, which are born randomly all over the seven continents.
This question focuses on medieval Europe, right after the fall of the Western Roman Empire and the establishment of feudalism, and of course on Nymphs and predators. So, let's get to the good stuff. What are Nymphs, and what do they have to do with predators?
Nymphs are manifestations of nature that form when excessive amounts of life force build-up; every time something dies, part of its soul stays behind, and so Nymphs often appear on or in mass graves, battlefields, and rain forests (lots of life, lots of death). They appear to be beautiful women in their prime, clad in clothing of leaves (if you thought of Tinker Bell and her friends, you have the right idea).
However, there are many differences. Nymphs are unaffected by heat and cold, being able to wade through lava flows and stroll through the northern wastes, can breathe air and water with equal ease, and cannot be poisoned or diseased. As for personality, Nymphs are naive, peaceful, and have a deep love and awe of nature. They have an equally strong belief in fate and feel that they came to life to fulfill a purpose, though they don't know what it is.
Additionally, the Nymphs have extensive latent knowledge; they can speak and understand any human language, and if they see a plant or animal, they not only recognize it but know how it behaves, how it survives, what it eats, so on and so forth (in the case of plants, they know the specific type of plant it is, what parts of edible, and other trivia).
After being "born," therefore, Nymphs start a journey: partially to see the wonders of nature, partially to find their purpose. However, there's just one problem. Being creatures of nature, Nymphs have a built-in ability to return to nature. How? By being eaten.
Seriously, if a Nymph who hasn't found a purpose yet encounters a predatory animal (AKA anything that can eat her) she'll not only allow it to eat her but encourage it to do so. This may seem counterintuitive, but A) since Nymphs are biologically immortal, this actually keeps the world in balance, and B) it's not the end for a Nymph.
After a Nymph is eaten, her potent natural energies enhance her consumer's attributes to supernatural levels, and her spirit enters its body, contributing its energy and intelligence to helping its "host" survive. For example, Tigers are ambush predators; stealthy, strong, fast for short distances. A Nymph-hosting tiger is supernaturally stealthy (becoming invisible in shadows) and can dent or even sunder plate armor with its paws. It's also somewhat faster and more endurant, being able to keep pace with and even outrun a horse.
TL; DR: How can an early medieval society find Nymphs before predators eat them and become almost unstoppable monsters?
Please Note: With intelligence, and proper recording (specifically plotting points on a map where Nymphs have been found) humans should realize a pattern and figure out that Nymphs appear in places where lots of things have died or been interred, like cemeteries and battlefields. Humans will then be able to post watchtowers in or put walls around these areas. However, many of these areas will be inherently dangerous areas in the wilderness, and humans will have to put more effort in to find those.
I've considered humans using smart birds like ravens to locate targets (AKA Nymphs), which according to this website can identify individual human faces, take pictures with cameras, and even place recording devices (technology is irrelevant to medieval Europe this example is meant to illustrate raven intelligence). In fact, I've even heard ravens can be taught to speak. However, I'm not sure how helpful they would be; how would ravens bring humans to the Nymphs they've found when A) Nymphs will likely keep moving after being "located" and B) they have to somehow guide the humans to the Nymphs; perhaps someone could spot the raven through a spyglass and have their comrades travel to its location?
As always, I appreciate your input and feedback; if there's any problems with the question, please let me know so I can fix/improve it and make better questions in the future. Thanks, everyone!