How did a large snail get into the toilet bowl?

If it came up the drain from outside there is obviously some hole or crack either in your drain or sewer!

Surely it is more likely to have crawled up the wall then fallen down into the toilet?

The real mystery is how it got into the house, but if the weather is warm it probably came in through an open door or window.

Could you or someone else inadvertently have brought it in on your clothes?

It is amazing where snails can get to. I once grew plants high up on a balcony and they managed to climb the walls.
I once had a garden worm in the toilet (not when I lived high up, but here at ground level).