The Silth Mouse dropped the Chrome Nanite Coating during the Mousehunt birthday event. Now it's a matter of pride (and trying to complete the full mouse list) to get hold of this mouse.
I've not caught mine yet, but do drop a comment on how many Wicked Gnarly Cheese pieces you used to finally catch one.
Successful hunters swear by the ASG and the Kraken for catching them.
This elusive mouse makes his home in the damp subterranean rivers of the underpass, hiding in rocky crags. Its bat-like squeak and the splash of its feet can often be heard in caverns, but when hunters approach, it dives into the water and swims away faster than a Burroughs trout. Because it has spent so much time in seclusion, these guardians of the underpass have unusual taste--anyone aiming to catch it should note that only the gnarliest cheese will tempt it out of hiding.