The People In Your World Have Never Read the Monster Manual

(L to R): Man, Child, Hobgoblin, some Pixie, and Lou Ferrigno (Artist)

Did you know this?  It has never occurred to me before. The people in your world have NO IDEA what 99% of monsters are because they never owned a copy of your Monster Manual and they don't have the Internet (probably)!

In fact, there are Men in your world who would see a gnome, and call it a dwarf.  And certainly there are elves who couldn't tell a Hobbit from a child, hairy feet or no.  So don't get me started on what, for instance, a Bullette is or Wyvern might be.  

Now how is this gameable?  The answer is, never describe a monster or even a race of common type NPC people by their names. Always describe them by the way they look, sound and smell.  If you want to, you can even conflate one creature with another.  A commoner might describe a purple worm, a wyvern, a giant snake and even some demons as "a dragon."  They can't tell you! In actual folklore, a giant, a troll, a titan, an ettin, an ogre, and so forth really all describe different kinds of the same creature, so there's even some real world precedent. If you're talking to any character who doesn't speak common, then even commonplace words become a problem.  If they see a dragon, they might have a completely alien word for it that you have never heard, and then it's up to the two of you to figure out whether you're actually both talking about a dragon or something else entirely!

Players are very good at absorbing information. Weaponized Asperger's is definitely A Thing in our hobby and you know it.  Using metagame knowledge is almost inevitable, no matter how hard we try to block it out. So do your very best to confound their desire to know EXACTLY what they are fighting when they meet a monster which their characters have never seen before!  It will make fights seem more dramatic, dangerous and exciting.  And rumors & conversations about the odd men and monsters enpeopling the Realm will seem more exotic as well.