Couple of examples from Dead Space. The second one's clearly just a full model swap, but the first one still uses morphs, particles, and flickering lights to hide any jarring transitions.
Taking a look at the rat example, my guess would be:
Bulging skin- morph targets
Mouth and eyes peeling- morph targets
Elongating teeth- scaling separate meshes
Split tail- model a closed tail, cut seams, pose in the open position, model the interiors
Limbs extending- probably bone scaling
Final result- entirely different model
And all of those closeups are most likely entirely different models built specifically to show the transformations in detail.