Hydra

The Hydra is a titanic beast of the mountains, whose ill-tempered and ravenous nature has proven the doom of many an unsuspecting morsel. Indeed, so ferocious is the Hydra that campaigning armies often make wide detours to avoid intruding on such a beast's bone-strewn lair. Those that march on regardless do so either out of confidence in their battle-prowess or ignorance of the Hydra's savagery. 1a

Only creatures as twisted as the Dark Elves would look upon the Hydra's vicious glory and deem it insufficient for the task at hand. But ever since the Witch King's Beastmasters first tamed the creatures, the Naggarothi have continually experimented with breeding techniques and dark enchantments to raise the raw ferocity and power of successive generations. The War Hydras of Naggaroth's armies are therefore a far more daunting foe than those the Dark Elves first encountered many long centuries ago in the Annulii Mountains. 1a

War Hydras are notoriously difficult for their handlers to control, and a Beastmaster must be quick with his lash lest he be devoured in the enemy's stead. They are even more difficult to slay, for not only are their scaly bodies incredibly well-armored, they also regenerate damage at a frightening rate. A foe's only chance is to sever all the monster's heads in quick succession -- if even a single one remains, the remainder will swiftly grow back and devour the impudent attacker for his troubles. 1a

There are many ways to be slain by a Hydra: torn apart, swallowed whole, crushed underfoot, immolated or even skewered by arrows as they ricochet off its scaled hide. Generals often see even their finest troops pulverized by just a single War Hydra and are then left to watch with horror as the beast emerges unscathed from the carnage, vents for an ear-splitting roar, and then charges forward with thundering strides to trample another regiment into ruin. Even worse, some are capable of breathing streams of fire from their many mouths. 1a

Source
Warhammer Armies: Dark Elves (8th Edition) -- p. 49