Hydra is a Grimoire card from the original Destiny (Destiny 1), found under Enemies → Vex. Grimoire cards held most of the game's lore and were read on Bungie.net and in the companion app, not in the game itself.
Card text
The Hydra is a miniature fortress. Despite its physical slowness, it is a rapid processor of the data fed to it by other Vex, and what it lacks in mobility it makes up for in impregnable defenses and rock-melting firepower.
Grimoire text written by Bungie, shown as it appears in the Destiny 1 game files.