Well, there are a few exceptions to the rule, because, you know, nothing in life is ever completely free (except for maybe your in-laws' advice, which is always free and always unsolicited). For instance, if your bag is over 50 pounds or 62 linear inches, you'll have to pay a fee. But let's be real, if you're trying to sneak a small elephant onto the plane, you're probably doing it wrong.
And then there's the special items - you know, like skis, golf clubs, and bicycles. Those things are like the cousins of the bag world - they're a little weird, but you still have to invite them to the party. Delta's got special rules for those items, so be sure to check the website before you try to sneak your unicycle onto the plane.
PPT - Delta Airlines Baggage Policy PowerPoint Presentation, free
Now, I know what you're thinking - what about emotional support animals? Well, let me tell you, Delta's got some strict rules about those adorable little furballs. You'll need to provide a doctor's note and all sorts of other paperwork, because, you know, they want to make sure your emotional support chicken isn't just a regular chicken in disguise.