Of all the horrible things that can happen to you in the bathroom, weighing yourself is not the worst. Sometimes, a creature can actually make it's way into the toilet bowl. This is a terrifying and simply gross occurrence. You don't have to live in a particularly pest-infested area for this to happen either. Sometimes proximity to a basement is enough. You do need to recognize the danger and know how to react.
Pests that can reasonably end up in your toilet bowl include rats. In fact, they can somewhat easily crawl into the empty pipe and make it into the bowl. Unfortunately, they may not be able to reverse course. Snakes have also been known to make their way into toilets, as have frogs and even squirrels, although squirrels usually enter through a vent pipe. Even if you don't end up with a bite in a sensitive area, the fright alone can be rather traumatic. Also, your children may be affected if this happens in your home.
Your immediate action should be to slam the toilet lid shut. Although some animals might be flushable, you may need to physically remove animals from your bowl. Unless you are an animal wrangler, you may want to call for help for most creatures, either from a plumber or an animal-control person. You should be able to manage frog and small-snake removal yourself, but rats carry disease, so they are best left to the professionals.
If you have several instances of animal infestation, you could cover the vent pipe with a wire mesh, which should keep out most pests. Rats and snakes may be burrowing close to your outside pipelines and finding a way in through breaks and cracks. You might need to have your system scoped with a camera to find out where your pipes need repair. Breaks need to be repaired anyway to save water or prevent contamination of your property.
Having pests in your home is distressing enough, but finding them in your toilet bowl is a truly awful experience. You may laugh about it later, but you certainly do not want a repeat of this event. You can take precautions to help prevent pest invasion in the future, but don't be afraid to ask for help in the short term. Rats, squirrels, and large snakes can be dangerous, so steer clear until the professionals arrive.
To look for a plumber near you, visit sites like http://terryrossplumbing.com/.