Nice gate, Fire-Man!
Yep, cattle panels. First hog I ever bought was about a 200lb boar I bought at auction.....The said "hmmmm. How am I getting this dude home in a Mazda pickup?". I had my sawzall, a roll of RedBand wire, and assorted "truck junk", so we went to the hardware store across the road, picked up some panels, and slapped together a pen plenty serviceable for the ride home. It's already a small bed and the tool box eats up a good bit of it, so we built it to fit with the tailgate down to pick up a little more square footage. At a later date, we spent a little time and a few bucks on hardware to make it prettier, stronger, and more convenient to install and remove. We used it for years, made a few bucks here and there hauling hogs for other people, and it worked great. Adapting a trailer would probably be even easier.