I think you are right...but what I found curious about that scene was that as the mouse is eaten it too becomes invisible...actually you should be able to follow the mouse (at least to some degree)as it makes its way down the gorilla's throat...a similar movie, "Memoirs of an Invisible Man" actually showed food (or was it a drink) as it made its way through the invisible man's digestive system.
The animal that ate the mouse WAS the gorilla. You can clearly see when the animal grabs the mouse that there are indentations in the mouse's fur indicating fingers. If it was a cat that ate the mouse, then the mouse would have been batted all around the cage first, because everybody knows cats like to play with their food first. And cat's usually bring their faces down to their prey, and start gnawing at it. They don't just go and straight bite their prey's head off.
Although a gorilla's diet consists mainly of vegetation, they may occasionally eat meat, insects and eggs if they are easy to obtain. A mouse would be an unlikely meal for a gorilla, but is not completely implausible.
1: It doesnt matter if it was a rat or a mouse.
2: It was a snake that ate the rat/mouse thing. you see the mouse/rat (herfore refered to as the rat) lifted off the ground and it is crushed, the same way a python kills. Then you see the snake witht he blood on its VERY distinictive teeth. Even with this in mind its pretty fake because pythons dont have teeth that big and they ingest things whole without biting them. Added for poingancy i guess.