they claimed to have found a mouse in the beer.
This is a quality control issue. Most soda and beer cans are turned upside down and then shot with a blast of air to clean them and make sure they are empty. Somehow when that can got flipped back arround, in the seconds it took before the beer was added and the top sealed on it, a fish got into the can, How? I dunno. But they owe you a refund and possibly need to review their quality control process.