The Pipe Snake is an enemy who appears in a cutscene in The Bathroom. Ned Needlemeyer is running through a drain pipe while the snake chases him and tries to eat him. The snake bites at him a couple times, but does not completely swallow him. Ned then manages to outrun the snake and escapes through an exit pipe.
This cutscene is shown if Ned falls through one of the toilets that the Clogging Trio inhabits (when they are not jumping out of the toilet).