There were three of them to begin with: Tuck, Hodge and Flitch. Three little pigs. When they were still quite young their mother drove the brothers out of the muddy wallow that had been their home, and they had to fend for themselves.
They got on well for a while, but then a garm-wolf picked up their trail and began to stalk them.
Tuck was killed by the garm-wolf, and Hodge and Flitch went on without their older and wiser brother. They might have been all right in the end, but something else happened to them on their travels: the two remaining Marrowbone Brothers got too close to an outbreak of werefire, and it changed them. It turned them into creatures of malice and hunger. It changed them from talking pigs into hogmen, vicious trolls with an appetite for human flesh. And it was in that form that Will Lightfoot encountered them in the sewers under the city of Skald.