Usually insects - such as various beetles and their larvae, as well as ants & termites etc - Even some reptiles have been known to live in rotten logs.
snake
It is a log that has become rotten.
grass
Nutrients, a place to grow, and protection
the rotten log eating insect (named dixie normus)
wild mushrooms get nutrient from it
You can find bugs in rotten wood.
Bacteria, various beetles and their larvae, cockroaches, centipedes, millipedes, etc.
If u mean slugs then they live in wet, moist areas such as under a rotten log.
squirell
in log cabins
Depending on the size of log cabin it could be anyone.