It will still be lurking when you finally breach that cavern, unless you happen to have an animal man tribe down there who can kill it. And it will eat your FPS while it lives due to the gas.
Build yourself a 3/7-deep bath between your fort proper and the caverns, get a dwarf with decent dodging and weapon skills (ideally with a spear in this case, though axes and swords are suitable as well) and full armor, preferably steel, and you should be alright. I've had dwarves solo forgotten beasts with around Proficient in axes, armor, shields, dodger, and fighter and equipped with bronze armor and axe made by a Proficient smith, so they aren't ludicrously tough unless you get one made from nonliving material, which this one isn't.
The only real danger of FBs is their great size, which can be compensated for with good equipment and skills, and their syndrome-causing ability, which are often not actually all that dangerous. The horror stories of FBs with gas that causes instant necrosis of the lungs are probably mostly true, but such creatures are rare. If you've got a legend with full masterwork steel as I've had in the past, it's just a matter of the dwarf not getting bowled over by the FB's charges all the time, which will happen eventually.