A floor, sides and at least one axle. I gathered some cows with a guy that had a half covered, single axle, 16 foot gooseneck that was the handiest trailer you could ever want. Because it was single axle you could turn around in the middle of a one lane road without getting a tire off. If you try that with a double axle you end up with a bent axle. It wouldn't be great for showing or hauling a long ways but if you have a cow roped and need to drag it in a trailer that's the ticket.
There's nothing like the Rockies in the springtime... Nothing like the freedom in the air... And there ain't nothing better than draggin calves to the fire and there's nothing like the smell of burning hair. -Brenn Hill