The Hydra is a serpent-like beast with nine heads (although in several myths the Hydra were described to have five heads or even more). The Hydra's nine heads are used to symbolise the number nine in the runic alphabet. 
Behind the scenes
- In Harry Potter and the Half-Blood Prince, Severus Snape describes the Dark Arts as though they were a creature similar to the classical depiction of a hydra: one that regrows two heads for every one that is cut off.
Notes and references
|Professor: Bathsheda Babbling|
|Textbooks: Advanced Rune Translation · Ancient Runes Made Easy · Magical Hieroglyphs and Logograms · Rune Dictionary · Spellman's Syllabary|
|Known Runes: Acromantula · Demiguise · Ehwaz · Eihwaz · Fwooper · Graphorn · Hydra · Quintaped · Runespoor · Salamander · Unicorn · Unknown · Mark of Merlin|