Might and Magic Wiki

The hydra is the fifth-tier creature of the Mountain faction in Heroes of Might and Magic I.

Their attack will damage all surrounding enemies, and is impossible to retaliate against. They're capable of doing a great deal of damage against enemy forces, but due to their low speed (only mountain troop with slow movement), they might be better for defending the town than accompanying the hero on the map. But when led by a hero with the Teleport spell, they are excellent at castle sieges; teleport them inside a two-space opening behind the walls and they can wreak havoc on defenders.

However, hydras are not cheap. In fact, they are the third most expensive unit in the game in terms of gold cost, surpassing the paladin, cyclops, and genie, which are of higher tier. Fortunately, they are quite sturdy, having the fourth highest hit points in the game.

Heroes of Might and Magic I creatures
Farm (Knight) units Peasant - Archer - Pikeman - Swordsman - Cavalry - Paladin
Plains (Barbarian) units Goblin - Orc - Wolf - Ogre - Troll - Cyclops
Forest (Sorceress) units Sprite - Dwarf - Elf - Druid - Unicorn - Phoenix
Mountain (Warlock) units Centaur - Gargoyle - Griffin - Minotaur - Hydra - Dragon
Neutral creatures Rogue - Nomad - Ghost - Genie