Greymon is a Legendary Digimon part of the Original Ones, a group of Digimon consisting of Greymon, Garurumon, Birdramon, Kabuterimon, Togemon, Ikkakumon, and Angemon, and Gatomon. Technically speaking, however, from Botamon to VictoryGreymon are all given a legendary status, as well as the Fresh to Final Stages of the other Digimon listed due to their importance in The Great Beginning, the Adventure timeline in other words. Everybody speaks of their grand power so much that their entire lines are worshipped like gods. The Adventure timeline goes down in legends of this Digital World as setting the foundation from which the Digital World was formed, and Greymon surely has an important place in the beginning chapters of Digimon. The helmet Greymon is best known to have is actually a Tyrannomon skull, with the 3 horns being a mutation due to wearing the skull as a helmet.
Digimon World: Bronze and Cobalt Versions
The backstory above explains why Agumon's entire line is so rare in B/C, as to give a more important feel to it than have it be more common and easy to find. Greymon is obtained as Botamon after achieving Legend Tamer rank, as the opponents for Legend Tamer are none other than the original 8 Digidestined themselves. Greymon is a Balance Digimon and of the Dragon species. It digivolves from Agumon at Level 17 and digivolves into 3 different Digimon depending on certain things; Into Vaccine MetalGreymon at Level 27, into Virus, or BlackMetalGreymon at Level 28 with 500 Dark Exp, or into SkullGreymon at Level 23 with 1000 Dark Exp.