In the Games
Digimon World 4
Dorumon is one of four Digimon you can play as, the others being Agumon, V-mon,
Digimon Story (Digimon World DS)
Dorumon is in Digimon Story, known as Digimon World DS in the US and can only be obtained
once in the game. You get Dorumon by first obtaining the right amount of Tamer Points
to receive Dorimon scan data. You can then evolve Dorimon into Dorumon.
Digimon Sunburst (Dawn) and Moonlight (Dusk)
I have not played these two sequels to Digimon Story yet so I don't know if he is
in both games, but he wasn't in any of the version exclusive lists so
I'd assume he is in both. I also do not know how to get him in these games, but once I
find out I will update this.