Awanezumi is notable for it being one of the only completely scrapped designs for Pokemon Black and White. It's even speculated by The Cutting Room Floor to be a placeholder for testing animations. This is due to their sprites looking more like full on spritesheets, with multiple different parts, which can be stitched together and form this cute mouse guy!

Judging by certain elements and coloration, it seems Awanezumi is a water type, and very mouse-like akin to Pikachu. I wouldn't be surprised if this was the original Pika Clone of Gen 5, predating Emolga, but it's hard to say given that it lacks the cheeks. Given how Gen 5 is a reboot of the series, I wouldn't be surprised if Awanezumbi took Marill's place in the games, had it made the final roster.