how can i make a bot/character that responds every few messages?

so, while ago, around a year or so, i was in a server where they had a bot of Sans, and the bot would just randomly say something every five messages, now the server doesn't exist and i couldn't ask how it was made, does anyone know how to do that? i suppose it's made with a bot or smt like that but I really don't know...

thanks! (and sorry if my english is kinda bad, it's my second language, and I'm not sure if i accidentally sounded rude)