a whole different beast

Something very different to, and often more difficult or complicated than, something else. You may think that your time as a TV show writer prepared you well for this project, but it's a whole different beast working on a film. It's one thing to babysit your friends' kids from time to time, but having your own children is a whole different beast! A: "I've been here 10 years already, so I don't think becoming a manager will be a big transition." B: "Are you serious? No, managing employees is a whole different beast."