They don't use the creature's body for resources, apparently. It's just sitting in the refuse pile outside. Took the haulers a good month or so to get it up there. Either way, it would have been worth about 234 if the coal was unrefined. I did the math.
I think the steam-made one residing in water for a whole season warrants more attention over the mud one, honestly. He died in about the same time span, but still, steam in water. The logic makes my brain sad.
I had another Forgotten Beast. This one was a humanoid made of Rutile that spat a "poisonous gas" about 2 squares in front of it, but didn't seem to do anything to the dwaves. It, too, took forever to kill, just like the coal beast. If anything, the poisonous gas was its blood or whatever you want to call it. The dwarves got nasty chemical burns on their bodies if it came into contact with their skin. Fortunately, they were all wearing full iron sets of armor.
An even stranger thing happened with this beastie. During the fight, my Swordmaster cut off both its arms, and somehow, it still managed to kill a hammerdwarf. According to the report, it pushed the dwarf (with what, I don't know), which sent his skull into his brain and killed him instantly. This is after who knows how many turns of him harmlessly punching them until his arms got hacked off.
Before the fight, it got into a tangle with a cat, as most invaders do. The cat managed to fracture and cut open it's right leg (the creature is still made of Rutile) before strangling the cat to death.
In case you are wondering, the Rutile Beast weighs 22999 dwarfpounds, and that's not considering the other body parts I can't find in the mess in the garbage dump. It, too, is worthless as a building material.