Ultra Kaiju Monster Rancher is only on the Nintendo Switch.
Learn how to use the Fusion Calculator app on LegendCup, for creating powerfully strong baby Kaiju . With just a few button clicks You can make perfect fusions, and get fusion suggestions, all without needing to know how the fusion mechanics actually work.
If you play Monster Rancher 1&2DX Also on the Nintendo Switch, there's other apps for these games as well to help master raising your virual pet monsters :)
If you're unfamiliar with the series, it is less of a monster taming/catching game -- and more of a fantasy pet breeding and raising sim. Your monsters have a finite lifespan and you must raise them as best you can with your available budget which you can increase by fighting and winning tournaments. There are also often many locked/hidden monsters that must use story progression to access.