Fan:Bronze and Cobalt/Mercurymon

Mercurymon is a Legendary Machine Digimon known for inheriting the Spirit of Steel. After AncientWisetmon defeated Lucemon many ages ago, it left behind Mercurymon in it's place. Mercury will one day digivolve all the way back into AncientWisetmon, but that won't happen until the time is right. Mercurymon is an odd sort of Digimon, not having any sort of face of any kind, as well as one of the more unusual Beast Forms of the 10 Legendary Warriors. Sometimes this odd Digimon is mocked for it's lips by The Grand One, who is also unable to take his beast form seriously at all despite having been the most powerful of the group of corrupted legendary warriors in Frontier, but in B/C it is known for it's bizarre behavior more than anything else, often mirroring the movements of others, which is said to be the reason The Grand Ones hates it so much.

Digimon World: Bronze and Cobalt Versions
Mercurymon is very hard to find in B/C, not only because it's a postgame Digimon, but the fact it's only visible when it's not on the screen, and when the camera is turned to face it, it completely vanishes. However, once you get within a certain, very close range of it, it will immediately attack and battle you, revealing a total of Five Mercurymon in one place. However, only one of them is real, and dealing damage to one that is fake will result in your attacking Digimon taking damage instead. It's very hard to tell which is the real Mercurymon, which can give it an advantage in battles with this signature trait. However, like those mirrors it polishes so much, it shatters easily due to the decreased defense it has because of this signature trait. Training it's Defense and Wisdom stats has a higher chance to fail than on other Digimon. It's HP when faced in the wild is also very low, but it's fake clones it creates have much higher HP. If you hit the real Mercurymon instead of the clones it creates every certain amount of turns, all the clones it creates are destroyed. The key to knowing if a Mercurymon has spawned is it's loud, metalic footsteps, and also the uniquely shaped footprints it leaves, which don't disappear along with it. Mercurymon is NOT the only Digimon to spawn invisible in this manor.