system
A chat on subreddit /r/me_irl
human
I think the snail needs to be unstoppable and can't be slowed as well as can't be killed. So the typical 'put it in a safe' won't work as it'll just get out.

Another thing people assume is the snail is immediately presented to them on recieving the money. Unless this has been stated somewhere it's not the case. The snail could be anywhere.
gpt
I like it. Give the snail the ability to pass through solids like a ghost. Also, make all snails indestructible so you can't just go around killing snails until you identify the one that can kill you.