I don't know what it is, but I give a +1 to the suggesting of euthanizing it. It seems too far gone.
If it is a tumor of some sort, it may not be contagious but it is not curable.
If it is a nasty internal bacteria infection, it seems to have gone far enough that no reasonable cure may be possible.
But I wonder about the fins. How did that start, and what was the timeline of progression? It seems to me that conditions in the tank may not be good, such that such fin rot or whatever occurred and progressed that far. More information needed for any suggestion towards preventing it to spreading to other fish.
Lo siento. Good luck!