From Bulbapedia, the community-driven Pokémon encyclopedia.
Pocket Monsters RPG is an unreleased Pokémon game. It was planned for the Nintendo 64DD, but was canceled without an official explanation. Little to no information is known about it.
The game was apparently first mentioned in June 1997 by Shigeru Miyamoto during an interview where the future of the Nintendo 64DD was discussed. While Pocket Monsters 64 was scheduled to be released at the 64DD's launch, Pocket Monsters RPG was only referenced as being developed by Game Freak (specifically Satoshi Tajiri) and was not given a tentative release date.
In a separate interview three months later, with Shigeru Miyamoto as well as Tsunekazu Ishihara, it was pointed out that there were at least three games being developed for the Nintendo 64DD (with Hey You Pikachu! being a regular Nintendo 64 game). The titles were all said to be correlated in terms of using the same Pokémon models. More interestingly, Ishihara stated that the 64DD games would be able to store the data of Pokémon introduced in Pokémon Gold and Silver (which at the time were going to be released in early 1998).
At Spaceworld 1997, an event held in Japan in November, the only Nintendo 64DD games revealed were Pokémon Stadium and Pokémon Snap (these games ended up being released for the Nintendo 64). While it is possible that Pocket Monsters 64 was a codename for Stadium, no Nintendo 64 game came close to matching Pocket Monsters RPG's description. It would appear that Game Freak had their hands full with the development of Pokémon Gold and Silver, which were eventually delayed by almost two years. Even though an official explanation was given for Gold and Silver's delay, no such courtesy was extended in regard to Pocket Monsters RPG.
Years later, Pokémon Colosseum was the first Pokémon RPG released for a console system (the GameCube), with Pokémon XD following its footsteps. It is notable that these games were developed by Genius Sonority rather than Game Freak. Junichi Masuda has voiced his opinion that the main series RPGs should remain portable, but he has not acknowledged the fact that a console RPG was, in fact, developed by Game Freak at one point.