The Daemon Whisperer sat in the train car, keeping guard over his latest acquisition. It would make a fine addition to the bestiary. Maybe it could be trained and someday fight alongside him as a hulking guard-beast. Sitting, thinking idly to himself, the Daemon Whisperer did not hear the droning of wings on the breeze.
A squadron of Plague Drones descended upon the daemon engine like a swarm of flies (very much like it in fact). One of the drones disgorged an additional passenger that had been cradled in its bony legs; The Shepherd dropped from the sky like a cannonball. When he landed on the top of the cage in all of his armoured foulness, the cage bars twisted and dented leaving an impression. With a solid blow from his hammer, he struck the lock off of Rotbelly's cage and turned to look at the Daemon Whisperer.
"This beast was born to be free; to thrive and multiply," he gurgled phlegmily, then turned to walk away.
Meanwhile, Rotbelly gently pushed open the cage door. Much to his surprise, it opened! The armoured man had saved him! He wagged a tongue from his gruesome maw and barked deeply showering the Daemon Whisperer in a fine spray of bile and venom. Rotbelly and The Shepherd leapt together from the moving train as the Daemon Whisperer's ashen bolts blazed by them. When they rolled to a stop, they only had to wait for a few minutes before plague drones arrived to collect them. Another life saved from torment, free to engage in the great cycle of life and decay.
Rotbelly looked at The Shepherd, and knew that he had good master now.
Rotbelly is made from a spare Maggoth head and feet. The body was made of Sculpey. Detail was added with green stuff and texture was added with Stirland Mud. While he counts as a Beast of Nurgle, I tend to think of him as a Plague Walrus.