Koei Tecmo has previously announced that the port of the first Monster Rancher would be released in Japan in 2019. Today, they have finally confirmed the final release date of the port. It will be available digitally for Nintendo Switch at the Japanese Nintendo eShop on December 19.
It will also be released earlier on iOS and Android mobile devices on November 28. However, this game will be a paid app that is only available in Japan.
You can also watch the official trailer for this Monster Rancher port for Nintendo Switch and mobile devices. An English localization has yet to be announced as of this writing.