Just found out about an animal that I either (a) have never heard of before, or (b) heard of at some point but I blocked the memory.
Well, why should I be alone in my dismay at this animal? Here, you can all now know about this:
In the Gulf of California there actually exists a critter, Cymothoa exigua, that targets a fish by infiltrating its gills and latching onto its tongue. It proceeds to not only consume the organ, but will then replace it with its own body, providing the fish with a new fully functioning tongue it uses (probably a bit begrudgingly) to grind food against tiny teeth on the roof of its mouth.
Read the whole thing, if you are in the mood to learn about (yet another) horrifying parasite life cycle.
This would be impossible to put in fiction. No one would ever believe it.