Nintendo and TNX are named as the developers of Rhythm Heaven Groove. TNX is the entertainment company associated with Tsunku, whose music and production have been central to the Rhythm Heaven series.
The partnership continues the series’ unusual blend of Nintendo game design and Tsunku-led musical direction. Earlier Rhythm Heaven releases also credited Nintendo alongside TNX, whose performers and creative network helped supply the series’ distinctive vocal tracks.
The alternatives are all real Japanese game developers, which makes this question easy to mix up. Camelot is known for Mario sports and Golden Sun, Monolith Soft for Xenoblade, and HAL Laboratory for Kirby and early Nintendo collaborations; none is the developer named for Groove.