Ancient Chinese Secret.
You can only use it in Autovista. Can't fire the gun or drive it. Originally it was going to be drivable but Turn 10 basically said: "The current limits to the regular cars are so vastly different from the warthog that it would require a lot of extra time, effort, and money to make it drivable like its Halo 4 counterpart, and if we would do it, we'd have to make it realistic, cause that's how we are."
You can however change the paint to arctic or desert camo, though!
Can't drive the '30s Bentley 8 Liter either, not sure why on that one though...
You can do all the regular autovista stuff with them though. Explore them, open up certain parts (Warthog hood comes off like a Jeep Wrangler hood), examine certain features (Warthog tires are virtually indestructible, fancy that?), and of course, start the engine.