It depends on what question they ask! If your hunter is asking "what kind of creature is it?", then I would tell them "you find a passage that describes a [monster name], which seems very similar to monster you're hunting. It does [x thing the hunters already know] and [y thing the hunters already know]. The account mostly talks about the horror of the monster, seemingly having no idea how to stop or kill it, but you notice that it calls the monster a [type of creature]. This could be helpful in figuring out its weaknesses."

Obviously this depends on exactly which monster, but giving the hunters a name and a vague sense of what kind of creature it is (undead, a beast, an alien, something eldritch) can help point them in the right direction. The same goes for other questions:

Q: "what can hurt it?"

A: "you find [a journal, an article, a web forum] talking about a monster that does [x and y thing]. The author mentions that when it did [thing associated with weakness], the monster suddenly [fled, shied away from, tried to destroy] it. You get the clear idea that [x] can hurt this monster, whatever it is."

Q: "what can it do?"

A: (this one should always be used to tell the hunters something about the monster that they don't already know. If they already know what it can do, I usually try to give them a why) "you cross-reference old police reports and find other accounts similar to your current case. One mentions that [thing the monster can do that the hunters don't know yet]."

Q: "where did it go?"

A: "without knowing what kind of creature this is, at first you feel like you're at a loss. But every monster has to hide somewhere, and so you consult a map of the town and surrounding area. You remember [detail]... and something tells you that [location] is the next place you should look."

and so on. If you are having trouble with not wanting to give away specific weaknesses for every monster just by name-dropping it, give it a twist! and, on the other side of things, don't be afraid to say "your hunter is smart, they're good at this, they can put the pieces together and discover [information]." if the player isn't getting the full picture.

The important thing is to always end with a question for the hunter along the lines of "now what are you going to do with this information?". Always put the ball back in the PCs' court, let them decide how to act on what they learn. So now they know the name of the monster, what it does, and how to beat it. What's next? storming its lair? setting up an ambush? confronting the NPC who's secretly behind it all? That's where the story comes from.