A while ago, I saw an interesting description of Wario's abilities in the Wario Land games. That being, that he's basically like a playable boss character rather than a traditional platformer protagonist. That not only is he a villain or...
Sableye is one that could work. It's greedy, purple, goblin-like, and has a gem that's similar to the ones in Wario Land 4.
Plus it's natural inclination towards searching for gems would make it a pokemon Wario would want to have around