Since the old rumors didn't seem to be accompanied by any images of it, here's my take on the PokéGod, Millenium:
Using the code provided by fansites to obtain this rare Pokémon would actually give the player a Typeless glitch Pokémon whose menu icon was a flower, though its movepool looked more deserving of a Flying-Type (it actually couldn't learn any Grass-Type moves at all, even by TM), so I made it a bird-like creature whose fanned wings make it resemble a flower.
It's visually based on the blue lotus, a flower that shares one of its names with the udumbara flower from Buddhist legend (also known as the 3,000 Year Flower, though I used 1,000 in the Pokédex entry to tie-in better with the PokéGod's name). It also draws inspiration from birds of paradise (which coincidentally share their name with a flower), extinct birds (such as Archeopteryx), as well as the Shangyang (a one-legged bird in Chinese mythology). The single eye is a reference to Yu-Gi-Oh!, a manga series that started the same year that Pokémon was released in Japan that I believe very well could've inspired this PokéGod's name (a misspelling of "millennium," as in Millennium Items).
I haven't settled on a Type, but I'm thinking Water/Flying or ???/Flying. (I wanted to make it Ghost/Flying, alluding the the udumbara's otherworldly nature, but there are no Ghost-Type moves in its movepool to support this decision -- unless we pretend that some of those Glitch-Type moves are actually Ghost-Type moves that were removed from the game.)
I may try to make an actual sprite of it sometime, but I don't know if I want to base it on this symmetrical image or it I want to try drawing it in a more interesting pose...