to turn a magic item into a creature. I want to have a bag of holding that can run around and eat things but I'm not sure how to make it. The only example I can find was a animated eryinies's rope in a Dungeon mission. Should I just use an animated object?
You could just use the PRPG intelligent magic items section - one of the abilities an item can have is the ability to sprout motive appendages ...
The intelligence of the item would take care of the rest.
Mind, if you want it to be a bit tougher, tacking animated object stats onto it would round it out. However, a sack is not particularly large...
Perhaps the bag is instead a sentient bag of devouring with a special purpose to Eat Stuff Belonging to Humanoids?