Formless Spawn

Formless Spawn (previously "...of Tsathoggua") seem to be like mini Shoggoths, though they can only alter their shape, not create organs etc.

In the RPG...

The Formless Spawn are immune to all weapons, even enchanted ones, though magic, fire, chemicals, electricity etc can hurt them. They can consume up to their SIZ in prey, wherein the victim takes 1 hit-point of damage on the first round, 2 hit-points on the second round, and so on. 

    The 7e Malleus Monstrorum has made a few changes to the Formless Spawn. Their STR used to be range from 1D6+6 to 6D6+6, whereas they are now +11. Using the greater size, that changes the average from 135 to 160. CON used to be a straight 3D6, regardless of size, whereas now it ranges from 2D6+4 to 4D6+4. For a larger specimen, that changes the average from 50 to 90. Both changes make sense.

    One change of note is that their regular attack is much stronger now. It used to just deal damage bonus as damage, whereas the new value is 2D6 plus damage-bonus. Using the larger size from both editions—each with +3D6 damage bonus—this changes the average damage dealt from 10 to 17. To put that into perspective, the Formless Spawn went from having a 50% chance of killing an average Investigator in a single hit, to 97%!

In a video game...

Like the other amorphous creatures on this list, the Formless Spawn would be pretty hard to animate. Similar shape-shifting creatures appear in the game Prey, though they are mostly animated like spiders, only using their shape-shifting ability to ambush unsuspecting victims.

In a film...

The Formless Spawn could be pretty cool in a film. I'm kind of picturing something along the lines of the Venom symbiote from Spider-Man 3, but much bigger.