My wife has a habit of sending me articles on all things strange and terrible. Her most recent find simply begs to be used in my AD&D game. Do you like freakish monsters? Well, how about a tongue parasite!
I'm thinking my version of this beastie is a nocturnal threat that crawls into the sleeping mouth of any small to man-sized mammal and, within a matter of hours, replaces the creature's tongue. The creature secretes a powerful narcotic to prevent the host from awakening during the process, though the DM might rule that a character is entitled to a saving throw vs. poison to awaken. If a character or his companions become aware of the creature, it is easily removed so long as the process hasn't been going on for more than an hour. If the character is allowed to remain asleep with the creature feeding on his tongue, the transformation is complete in 1d3 hours. The character still has all normal functions and can speak, taste, etc. However, anyone seeing the beast is likely to have a very bad reaction to it. As such, whenever the character speaks, there is a 1 in 3 chance that the person he is speaking with will see the horrible creature. Most normal folk will flee in terror, but others may think the character is a demon or shapeshifter.
A remove disease spell will cause the parasite to wither, die, and fall out of the character's mouth, but his tongue does not regrow; the parasite devoured it.