Throat Leech

The Throat Leech is a small leech that infests ponds and streams. So small, it can swim down the throat of anyone drinking the water so it can attach itself to the inside of the throat. This process incidentally suffocates the victim as the leech swells up. Think of this little bugger as the Old World Rot Grub.

They're actually... real. When the Directoire sent (or allowed) General Napoleon Bonaparte - yes, that Napoleon - to invade the Levant, and after the Corsican hero had led his troops to failure (as usual); on his way back the troops were understandably thirsty and drank up some of the Nile Delta water. ACK.

A mainstay in first-edition Advanced Dungeons & Dragons Gotcha Monstery, editor Ian Livingstone set this little bastard in his White Dwarf's Fiend Factory series (as a Brit, he had a long memory); whence it swam over to the Fiend Folio. They appeared in 5e's Tomb of Annihilation, where, because 5e's designers are pussies, they have been downgraded to microscopic parasites that give you the exhaustion status effect.

• 
  The classic "throat leech in the canteen" prank