ZenithGreymon is a paradox-like entity with how it exists. It's designed was one designed for a homebrew hack for a completely different game unrelated to Digimon, and the design for the creature was based heavily on Greymon due to the creator's own fascination of Greymon-species Digimon. In the hack it was originally appeared in, it was a creature called "Megasuchus", in which it's game was a Shin Megami Tensei sort of game featuring Dinosaur-like creatures. Later, when the same hacker silently patched an MMO with ZenithGreymon's existence, it led to a lot of confusion to Digimon fans who first got a glimpse of the creature. While yes, it did indeed resemble a Greymon species Digimon, and even had the name of one, it was rather odd how basic it's design was for a Mega Level Digimon. Another odd quirk about ZenithGreymon is that every time it wins a battle, it showcases a massive ego, delivers a smug look towards the screen, with the text below it reading "Still the best. 1999" Other odd traits include the fact that ZenithGreymon has a jet-pack of sorts that doubles as a cannon, in addition to having both arms being cybernetic in nature. It's arms could also extend blades that invoke the image of Growlmon, but with the damaging properties of Andromon's Spiral Sword. It also apparently once worked for Belphemon at one point. The oddest part about it, though, is that nobody has yet to identify the original creator of ZenithGreymon, and it's existence is forever a mystery given all the odd quirks about it. Due to being relatively unchanged from it's original incarnation, as in, no changes at all, Megasuchus's abilities translated over to ZenithGreymon resulted in a very powerful Mega Level Digimon. It could digivolve from a variety of Digimon, much like with the older V-Pets, and it's attacks had a very immense base power to the point it could easily take on higher level opponents, and solo entire boss fights if it had wanted to. In it's original hack, it's overpowered abilities quickly proved to be a necessity as the game threw much harder opponents at it regularly, and in Digimon it is no different; all opponents, regardless of their level or attributes, will mindless gang up on ZenithGreymon without warning, as it's mere presence is a high threat to any NPC Digimon opponents in the game.
Digimon World (Fanfic)
ZenithGreymon debuts as one of the Mega forms obtained by the partner Digimon. Digivolving from MegaSeadramon when the requirements are normally in favor of MetalSeadramon, ZenithGreymon proves to be a very powerful partner Digimon. As with all major form changes, however, ZenithGreymon alters the personality of the partner, giving it a huge ego and the above mentioned quirk of "Still the best 1999", except in the actual text he only says it once, whereas it's implied he says it every other battle as well. Oddly, sometimes when he mentions 1999, he doesn't necessarily refer to Digimon Adventure; he is in fact referring to the date in which ZenithGreymon was first designed and created for usage in another game. Another quirk exclusive to this story is that ZenithGreymon can access Grey Lord's Mansion despite being a Vaccine Digimon, but cannot access Ice Sanctuary at the same time. ZenithGreymon is only accessed once, but the protagonist later acquires an item that allows instant digivolution into ZenithGreymon, which is called the "Hoax Card", a card that can also be sold for 1,500 Merit Points if one does not wish to use it for digivolution.