Bob the Mouse is from the wrong side of the tracks, a true scrapper from the bad side of town but how do you show this easily? Add some notches to his ears for that tried and tested "been through it all" indicator. The ol' 'chunk out of the ear' character design trick has been around for some time and most commonly on animated Funny Animal characters (though can appear on any anthropomorphic character including Petting Zoo People) it tells the audience very quickly if a character is tough or evil and generally different from the notchless characters. It usually pops up for characters within the rodent subspecies but can also appear on cats and other species with ears large enough to show it off. Sometimes can be used for added character in a Cute Little Fangs sense where the character isn't at all gritty and/or bad. Tangentially related to Good Scars, Evil Scars.
Examples:Anime And Manga
More examples are needed, might already exist