My first, and most memorable one was Imi, a great blob with wings. It first appeared about a year after breaching the caverns, terrified the dorfs that met it and seemed sure to send the fortress to its early doom... but instead it flew away into an upper cavern and eventually ended up in a HUGE underground lake. There, the brave Serpent Woman Spearwoman Kaslalrongquan engaged the beast, driving it back and wounding it, but it kept fighting back with its deadly spittle and evil-looking wings... and so they would remain, locked in deadly combat, for more than two years. During this time, a great project was devised that would open a hole in the above cavern, channel the water into a holed-out area below and thus effectively drain the entire lake into an even lower cavern, allowing speardwarves, swordsdwarves and marksdwarves in to attack and hopefully defeat the beast. Alas, it was not to be, and after a year of strenous work that saw two miners dead, three severely injured and a large section of the flooring collapse to 10 z-levels below, the project was abandoned and it was thought that maybe the beast and his adversary would simply remain in eternal combat, as a sort of symbol of the eternal struggle all dwarves must face in the trials and tribulations of their daily lives.
But lo! One day, the serpentwoman suddenly and unexpectedly slew the beast, and sent its corpse flying into a wall. She was hailed a hero, and became an eternal friend of the Mountainhome for her heroic actions. The beast's humongous body (about 11,000 units of mass in all) was turned into biscuits, stews, roast and other delicious things, or simply eaten raw; the sheer value of the beast's now-deceased corpse and its parts made up the bulk of exports for over four years, with a single meal made from its remains valued in the hundreds or thousands of dwarfbucks, and there being literally 11,000 pieces of it to cook, the supply was nearly limitless. Other beasts came and went, including those made of rutile (with deadly dust akin to a cave collapse- took half a year to defeat), blood (died in one hit), and snow (died instantly from it being too warm... ...), but none was remembered so fondly as the great Imi, source of the fortress' newfound wealth, its fame and most of its meals, and a legend was already beginning to form around it and its deeds, and of the mighty prowess of she who would after a battle worthy of Armok himself, slay it; unfortunately, it was cut short by a goblin invasion, a partial submersion of the fortress and a giant troll breaking down the last parts of doors that kept the unwary out, submitting them and all of Tomeauthored to a fate reserved for those who fall in epics, and being a great story still for the surviving parts of their civilization that would later find it, scattered and broken, and of the axelady master that returned in fury to avenge her brethren after an epic journey across the very tops of a world-spanning mountain range, only to be cut down by the nefarious lash of a goblin master, and reduced to a pulp by his evil troll masters and sword-wielding kin soon thereafter; however, none of this bothered the ones now taken up residence in the fort, who boasted of their might, power and ability to defeat the foul bearded ones... until a day much later, when a dark shadow rose from the neglected caverns below them... but that is a story for another time, and indeed other ears than were now so vain and ignorant as to think to ignore that which does under the living earth lie... in watching, and in wait.
Best part?
It was composed of vomit.