The Weather Demon is a mythical energy beast who creates rotten weather.
In Muppet Babies issue 10, Bunsen and Beaker invent a weather machine so that the Babies can turn off the rain and have a picnic. Baby Animal steals the machine, and starts pressing buttons, apparently causing a snowstorm, a typhoon and a hurricane. It turns out that the weather machine doesn't work, and that the Weather Demon is actually creating the bad weather.
The day is saved when the Weather Demon makes friends with Animal, who he resembles. The Demon creates a rainbow that carries the Babies home to the nursery.