"Yeah, I got your Zerg right here..." -Terran Marine from the Demolition Team on the Amerigo, shortly before a Hydralisk abducts him
The Zerg Hydralisk is a warrior strain that forms the core of the Zerg armies. They serve as the primary ranged unit for the Swarm, supporting the Zerglings and Ultralisks by shredding enemies from afar.
The Hydralisk was created from the Slothiens, peaceful caterpillar-like herbivores. When the Slothiens were assimilated, they were perverted into nightmarish serpentine predators. They had gained hooked claws for movement and combat, double-jawed maws filled with sharp teeth, and a spit attack that fired Needle Spines from their shoulders. These spines could fly like crossbow quarrels and shred steel. Hydralisks were often at the spearhead of attacks on the Zerg's enemies.
Tactics
Using Hydralisks: Hydralisks can pelt enemies from afar with their deadly Needle Spine attacks, making them a boon to Zerglings when attacking enemy armies. And since they use ranged attacks, they are invaluable for shooting down aircraft.
Fighting Hydralisks: Hydras can shred most lightly armored units and take out aircraft, so it would be wise to attack en masse or with lots of armored units, like the Terran Goliath or Protoss Dragoon. A good tactic for the Protoss is to use the Protoss Corsair's Disruption Web to prevent Hydralisks from attacking, then sending in Dragoons or Reavers to wipe them out.
Stats
Armor: Moderate
Weapon: Needle Spines. Ranged weapon. Average attack speed and damage.
Speed: Average. Slightly slower than the Zergling.
Upgrades: Burrow: Allows all Zerg ground units (except the Ultralisk) to burrow underground and hide(Hatchery), Ranged Attacks: Increase damage for Hydralisks(Evolution Chamber), Carapace: Increase armor strength for ground units(Evolution Chamber), Muscular Augments: Boosts Hydralisk movement speed(Hydralisk Den), Grooved Spines: Increase the maximum attack range of Hydralisks(Hydralisk Den), Lurker Morph: Allows Hydralisks to become Lurkers, Zerg units that attack while burrowed(Hydralisk Den)
Strong Against: Infantry, most Aircraft
Weak Against: Area-of-Effect attacks(Siege Tanks, Reavers, Guardians, Archons, etc.), Corsair Distruptor Webs and Dark Archon Maestroms.