Hydra

Hydras are level 80 Summoning familiars which have two heads. Hydras can be used for Ranged training, as their skill focus is Ranged. These familiars are also helpful during Farming and Woodcutting, as they have the ability to instantly regrow trees from Farming tree stumps.

Hydras attack with both Ranged and melee, depending on how far away the enemy is. The melee attack involves biting at the enemy with a snap of the head while the Ranged attack looks like a red dart.

Hydra pouch
Hydra pouch is made by using a Summoning pouch on a Summoning obelisk with 128 spirit shards, a Green charm and a water orb in the inventory. Making the pouch earns 140.8 experience points. Using the pouch to summon a Hydra gains 1.6 experience points, and costs 9 Summoning points.

Regrowth scroll
Regrowth scroll enables the use of the Regrowth special move for a Hydra. Using the Hydra pouch on a Summoning obelisk creates 10 scrolls.

Regrowth
Regrowth is a special move for Hydras. When used on a farming tree stump, it automatically regrows the associated tree without delay. Note that it can only be used on trees grown using the Farming skill. Regrowth is very useful when cutting Magic or Yew trees that a player has farmed, due to their significant respawn times.

Trivia

 * This refers to the Lernaean Hydra, a mythological serpent that had nine heads. When one head was lopped off, two would grow in its place. The hero Heracles (also known as Hercules), had to kill the Hydra as part of his Twelve Labours; he succeeded by cutting off its heads and quickly cauterizing the necks with a torch. The last head was invincible, so he finished it off by burying it under a boulder.
 * The Hydra used to require 60 summoning to use, but it was reassigned to 80.