Lost World: Shikaban

One night, Ryosuke Jagata is murdered by a thief after a hidden jewel on his person. His butler and rabbit boy Mimio (Mimi-chan) take quick notice. They soon butt heads with Mustachio, a private detective, who decides to take up the case. Mustachio, wanting more information, comes to the laboratory of Prof. Hoyoku Shikishima, a scientist experimenting with giving animals human souls.

Meanwhile, the planet Mamango makes the rounds. Said to be a part of the Earth from long ago, it is now steadily coming closer toward the Earth... Shikishima is researching this planet, and what Jagata was murdered over may be connected to this.

(Source: MangaDex)