Hell Leech

While not identical, it seems that Hell Leeches are a reworking of Watchers from the old Malleus Monstrorum. They are both giant leech-like creatures that dwell in murky lakes and rivers. Unlike the image below (it was the closest representation I could find), they both also have tentacles extending from near their mouths and both have paralysing poison.

In the RPG...

Though ostensibly the same as Watchers, a lot of the stats have been changed. For a start, they've been shrunk down quite a lot. Their STR and SIZ went from 140 and 225 to 85 and 65, so they're human-sized now. This makes sense considering that they are a leech. I can't imagine a creature with 225 SIZ would latch on to someone so much as swallow them whole. Their CON has also decreased from 130 to 85, again bringing them within human range. Another significant change is that they went from being immune to fire to taking double damage from it. This one makes sense to me as well. I'm not sure why they were immune to it in the first place. Due to its huge size and subsequent damage bonus, the Watchers' bite used to deal an average of 17 damage, whereas now that Hell Leeches are smaller, it only does 5, though blood drain following such a bite hasn't changed—averaging 10 STR drained per round. One last change is that due to their size, they used to have 35 hit-points, which is a lot, but now they are a more manageable 15.

In a video game...

Now that they're no longer the hulking beasts they were, I think Hell Leeches would work well in a video game. They're suitably creepy and no longer that difficult to kill.

In a film...

It could just be me, but a giant leech film just reeks of Syfy Original. It possibly could be done with seriousness, but I doubt it would be worth it.