It's kinda like the "secret boss" stage of TI- there's Fukurokumaru one too- there's small chance to trigger it while winning in TI and those suppose to give some extra moon coins for defeating them.

Well, to be honest I didn't have occasion to confirm on what exact reward it gives- because the 2 times that I had it triggered- was when I had all rewarding tries used up already and was helping group mate- and that info about moon coins is from him. Thanks for clarification!

Either way- it's just some tiny boss battle with small reward~