Theory: Wario was a Farmer

Abstract
Wario was a farmer. At least during Wario Land II, Wario had a farm and a love for looking after his pet chicken called Hen.

Arguments

 * Argument 1: Wario has a pet hen he loves just as much or even more than coins. In Wario Land II, he put his quest for the coins on hold to rescue his pet hen, who got lost, in Chapter 2 Level 1 "Return the hen to her nest".


 * Argument 2: In the intro of Wario Land II, the birds outside Wario's Castle could be chickens. They fly like birds, but so do the enemies literally called Chicken in Super Mario Land.


 * Argument 3: In Game and Watch Gallery 4, if you leave your cursor over the Fire Attack menu button for a few seconds, a cutscene plays showing Wario farming with a hoe and a chicken (presumably Hen).


 * Argument 4: The Dragonfly, Wario's unlockable Kart in Mario Kart DS, looks like a tractor.