Some ideas I had while paging through the 5e Monster Manual (which I'm quite impressed with, I may have to write a review later.)
- The basilisk is the female of the species. The cockatrice is male. To stumble upon them as they mate is not a pleasant experience.
- There once lived an albino red dragon whose favorite amusement was battling wizards who, believing him to be a white dragon, had filled their minds with fire spells.
- Erelhei-Cinlu and Menzoberranzan are the drow equivalent of the (ahem) Deep South. While dark elf society is decadent and duplicitous, other drow mock their excessive devotion to Lolth and cultural obsession with slavery.
- Dryads love to ride on the shoulders of treants. It makes them feel safe.
- Gargoyles only move when no one is watching... or so they believe; they are in fact rather poor judges of other creatures' visual acuity.
- All genies are able to grant wishes, but only nobles are authorized to do so.
- Ghouls and ghasts are actually a living race, not a type of undead, they merely recoil from holiness as if they were.
- Ice devils are not truly devils, but have lived in the nine hells so long as to work their way into the hierarchy anyhow.
- It is a mercy that the Invisible Stalker cannot be seen, and an even greater one that enchantments to see the invisible relay only a vague, formless outline. Those under the effects of truesight who have seen an Invisible Stalker uncloaked invariably retreat beyond the farthest shores of madness.
- "Blibdoolpoolp" is just an approximation of the name of the Kuo-Toa's lobster-headed goddess. Her real name is pronounced identically to the last three bubbles of air that leave a drowning man's lungs.
- In the far north, the deathless creature we call a mummy is known as a draugr.
- A shadow that impresses Orcus may become a shadow demon.
- All wraiths are bound to a magical item they coveted in life, all wights are bound to the tomb in which they were buried, and all spectres to a particular bloodline they wish to exterminate.