I've checked and it seems hydras only attack one target (with multiple attacks though).
What I heard though is undead sperm whales, but the KoB has to be at least semi-frequent, no?
Trees are the King of Plants. A non-animal can't be a beast (in the form that means animal/monster)!
If we're looking for an animal, I am sure it will present itself soon enough.
For the sake of making at least a sort of prediction, I'm setting up fights in the arena. A Hydra has been fighting a Bronze Colossus for minutes. The bronze colossus can't hit, the hydra can't damage. Ah, the hydra finally died, the colossus needed a lucky hit, then it was set.
The zombie giant sperm whale tore the BC up in seconds. They're dangerous, but rare.
The dragon kills the giant sperm whale zombie with fire. Maybe fire enemies are the new killers? Fire spreads again. It kills trees too!
I'll run a few tests then update.
Update1:
Bronze Colossus has melted. I accidentally made an Undead bronze colossus, which killed the dragon easy, but I guess that should never happen.
The GCS, even in a group has trouble killing the dragon, as expected. It seems fire may be the key. Time to test it against the dwarves.
Update2:
The dwarves lost. When fighting on grass, the area went in blaze. The trees are mostly intact, some of them lost their upper leaves and branches, but their trunks are the same. Might be a bug?
They fought 4v1, with full steel armour, steel shield. They had swords. Proficient in relevant skills (dodge, shield, armour, sword, fighter, discipline). Morale was off (and I believe I might have dwarves modded for NOFEAR anyway, dunno why).
I had a second group with crossbows who actually were looking good, 132 pages of combat needed to kill them (no difference in skills, just that they had crossbows and fought on non-flammable terrain). They didn't use their crossbows though, they just kicked and punched. Idiots. I'll repeat that test, but I can see dragons being the newest ruler in town.
Then again, this IS a fully grown dragon as it's the arena.
Update3:
For some reason I can't recall, I had dwarves with natural legendary skills IN EVERYTHING.
And even with they used their ADAMANTINE weapons/armour, they lost most of the time (main exception was a guy with a shield and a stack of 100 bolts, he just went there and stabbed the bolts through the dragon's skull and brain).
So yeah. Rerunning tests with basic dwarves.
Test results:
1 dragon vs Grandmaster Hammerdwarves (5 dwarf squad, grand master: shield,armour,dodge,fighter,observer,tracker,discipline,hammer, full steel)
New dragon, new dwarves, equipment and skills same.
Dwarves defeated 3 dragons in a row, ended tests.
1 dragon vs grandmaster axedwarves (5 dwarf squad, same as above, exchange hammer for axe)
Dwarves won again.
Looking over results so far: If you fight on flammable ground, you will be burned. Otherwise, with sufficient skill and an advantage in numbers, you can win. Blunt seems to get a better time (easier to get the dragon unconscious). 1v1... well, it seems it's more up to luck. You'll want more fodder and more hits.
Time to see how morale affects the outcome (I removed all modifications to creature_standard now.)
Update4:
Groups of axedwarves like before, with morale on.
Morale affects the outcome greatly, thought Discipline mitigates it heavily.
With Grandmaster Discipline: The axedwarves win easy, though 1-2 run off occasionally, the rest keep the dragon down.
With no Discipline: The axedwarves ran away instantly. 2 died, the rest got away, the dragon is bathing in lava and not giving a damn about going back and eating the rest.
With Proficient Discipline: I saw a single one of them run away, then he didn't join back into the attack. I'm guessing another one was holding back, but the smoke covered it. The dwarves lost.
Enough for now. My tests weren't very comprehensive (didn't repeat them enough), but I'd say dragons are great contenders to the throne of the King of Beasts. The real player though? Fear. Better level your soldier's discipline, people! Doesn't matter what the enemy is if fear is paralyzing your men.
It was inevitable.
TL;DR: Dragons, Fire, Fear. Discipline is extremely important for military. These are the main contenders for KoB title in v0.40 besides trees.