Yuggs

Yuggs are large, aquatic, pale grey worm-like creatures up to four feet in diameter and twelve or more feet in length. Yuggs have a large, round sucker-mouth similar to a lamprey or hagfish. This mouth contains several rows of hornlike teeth and is surrounded by six small tentacles six inches to two feet in length. Like snails, they leave stinking trails of slime behind them.

In the RPG...

Yuggs are fairly simple creatures. They have 3-point armour, with no special immunities, and their only form of attack is to bite and hold on to their victim. This attachment does drain an average of 22 STR per round, however, so it's nothing to be sneezed at (it's only just occurred to me what an odd saying that is). Unlike every other creature in the book, to break free from a Yugg, the victim only needs to oppose the 60 STR of the teeth/tentacles rather than the Yugg's own 125 STR. This still only has a 31% chance of succeeding on average, but at least it's better than the 14% it would be otherwise.

    The 7e Malleus Monstrorum has changed the STR, CON, and INT of the Yugg from the old edition. All the averages are the same. Only the ranges have changed.

    Just out of curiosity, I compared the Yugg's stats to the similar Hell Leech, and the main difference is that the Hell Leech is about half as small (with lesser STR). The Hell Leech is also more animalistic, with a mere 15 INT and swims twice as fast as a Yugg. As far as their attacks go, they both have the same attack chance (50%), but both the Yugg's bite and it's blood drain do twice the damage as the Hell Leech's. The only advantage the Hell Leech has is that its bite can also paralyse its victim.

In a video game...

Though primarily aquatic, Yuggs can come onto land (though with only 2 MOV), and without any strange immunities they are easy enough to kill, so I think they would work just fine in a game. The only caveat is that the Malleus Monstrorum says Yuggs dwell almost exclusively in the Pacific Ocean, so the game would have to be set either on or around it, but considering how many continents are adjacent to the Pacific, I don't think this would be that much of an issue.

In a film...

I don't think a creature that boils down to being a killer leech would be that scary in a horror film. And besides, it's been done (poorly) before...