Theory: Wario still has Hen

Abstract
Wario still has his pet, Hen, from Wario Land II. She now lives at his house in Diamond City.

Arguments

 * Argument 1: Hen was Wario’s pet in Wario Land II. She resided in a nest near Wario’s Castle in Wario’s Kingdom during this period. She also appeared in Game & Watch Gallery 4’s Fire Attack: Modern Version. After this point however, she didn’t appear in any more games, most likely due to the fact that Wario no longer resides at his castle, but rather in his Diamond City Home.


 * Argument 2: As explained in the Wario Land II manual by Wario himself, she’s his beloved pet, one of the few others that Wario truly cares for. It doesn’t make much sense that he’d be separated from her after taking up residence in Diamond City.


 * Argument 3: In WarioWare: Gold, Wario’s House received a slight redesign. In this redesign a nest was added to the roof above Wario’s Garage. Not only is this nest perfect in size for Hen (when compared to the Wario Land II nest), it also contains three eggs, which Hen is shown to frequently lay during her previous appearances.