most people in our world had never heard of a lion before globalization brought on by technology. Even today, i bet there are people in less developed countries who don't know what alligators or electric eels are. It's not unreasonable to assume that an adventurer might have knowledge like this, but it should just be given away without a roll.
I have to disagree with your statement about lion-knowledge. They are mentioned over a hundred times in the Bible, and even before the printed Bible, Christian oral tradition spread familiarity with the beast far and wide across Europe. Lions appear on crests and coats-of-arms all over the place, dating back long before anything resembling modern technology. In short, you vastly underestimate the power of oral tradition.
In the case of the electric eel or, say, the playpus, those are creatures from faraway lands. In D&D terms, they are more the equivalent of something like a couatl.
I assume the trolls in your campaign haven't been suddenly imported to the party's environs from halfway around the world. In most campaigns I've experienced, the trolls plaguing the nearby mountains have been doing so for quite some time. Even though few people have actually seen one, most people know they are there and what they are capable of, which is why no one goes into the mountains without a very compelling reason.
I'm not saying the PCs should automatically know every detail about every monster. But the example given in this thread, the troll, is not nearly exotic enough to require any sort of knowledge check IMO, unless it truly is a world where absolutely no one has ever seen one, and even the bards know no tales of hero-vs-troll derring-do.