Thread necro alert - found this while trying to figure out what to do with a FB that has been giving me problems. The beastie in question is a bit incongruous - An enormous tick made of feather, with deadly dust frozen extract.
My initial encounter was rather ugly, lost 4 legendary dwarven militia members. The battle lasted quite some time, and the beast had an awful lot of red around it at the end, but still seemed lively. I had taken the precaution of erecting a gateway bridge around that area in advance, so I managed to retreat the survivors and seal it off. I then fed it a variety of prisoners via mass-caging to see what would happen, gradually scaling up the number and intensity of foes, wondering if I could kill it and my foolish enemies simultaneously. Progressing over a number of years, I moved from goblins, to troglodyte quads (using masterwork mod btw), to ogres, to golems, cave dragons,magma hounds (verified that magma doesnt hurt it - just sets it on fire, apparently forever....making the next suckers who fought it that much more doomed). Finally, I created a passage to the HFS, with an airlock-style entrance, and introduced my FB to the clowns to see what would happen. It is curently happily engaged with about a dozen of them with no sign of tiring .
It seems I will have to create an obsidian statue out of this horrible feather monstrosity. I have already dug the appropriate water channels, and it is already right next to magma. Just confirming to anyone else that they are indeed hostile to the clowns, and that if it seems unkillable, it probably is (damage does not seem to accumulate after a point) .
Most importantly, in case anyone else needs to know, things made of feather are unkillable. I could find no reference to a forgotten beast made of feather via google or a forum search, so offering my contribution to !!SCIENCE!! , bought at the cost of some brave dwarves who had had more than 100 kills before being tickled to death by a feather tick. Man, what a mental image....