I've heard terrifying news about hydras able to rip apart multiple enemies at once.
The thing is, having hydras turn out to be total murder machines isn't at all unexpected, even from the perspective of an outsider to the DF community. The novelty of the King of Beasts is usually that it's an been a series of extremely improbable menaces with each new KoB being more ludicrous than the next. elephants in real life are huge, but not hyper-aggressive like they were in 40d, nobody expected a common fish to be a brutal aquatic killer, and after having a completely sessile aquatic organism being the most dangerous enemy a dwarf could face in the previous version, I'm not sure anything in .40 will top that.
Elephants? Unlikely, but possible.
Carp? Not unheard of.
Giant Sponges? They're large enough that when they sway due to currents, they can hurt.
The most obvious choice for the KoB of DF2014 is THE TREE.
It seems unmoving but it is the most wise, most dangerous enemy ever to exist.
It sits passively, waiting for its prey. Primarily birds.
It then creates a momentary blinding flash, stunning any flying creature, causing them to slam right into them.
Upon their inevitable demise, if left alone long enough, it eats the corpse with its roots.
The above-ground part of the tree is like the Lower Body of standard beasts, with the roots being the Upper Body. It's backwards, yes, but these aren't our Earth-trees.
Their brain, lungs, stomach and heart are all within the roots; they are spread out. Destruction of any part of the root system counts as "denting the brain", causing instant death of the tree - to prevent its corpse from being fed upon in this scenario, it evaporates.
The above-ground portion contains the other organs - the guts, liver, kidneys, spleen, and pancreas.
Removal of this portion prevents it from fully processing energy, killing the roots.
A rundown of Dwarf Fortress tree biology:
Instead of chlorophyl, these trees have eyes. Tiny eyes. MILLIONS TO BILLIONS TO TRILLIONS OF TINY EYES.
Once the collective hive-eye detects prey, they signal the tree-spleen to emit a light pulse (spleens are otherwise useless in Dwarf Fortress), blinding the flying creature long enough for it to collide with the hard epidermis of the tree. The flying creatures don't always die, but it's a team effort - if one tree fails to kill a creature, another may finish the job.
Upon the death of something near its roots, it lets the decomposers contribute it to the soil, and it feeds off the nutrients that way.
If said body lands directly on its roots, it absorbs the nutrients directly.
Either way, the nutrients go through the roots into the stomach, where some energy is absorbed. Said nutrients travel to the guts and get the majority of their energy absorbed there. The remaining sludge, which is often partly the remains of the decomposers, enters the liver, which processes it into more tiny eye components.
The energy itself goes to the heart, which distributes the nutrients properly, permitting further growth.
tl;dr: Toady One made the trees too hardcore.
Also, want to know why adventurers typically fail at grabbing branches when falling near trees?
Blinding flashes.
You seem like food to them.