All 4 Uses

Hydra
in
The Hunchback of Notre Dame
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This hydra of towers, giant guardian of Paris, with its four and twenty heads, always erect, with its monstrous haunches, loaded or scaled with slates, and all streaming with metallic reflections, terminated with wonderful effect the configuration of the Town towards the west.†
Chpt 1.3.2
The hydra of the Romancero and some other hybrid forms, the Vedas and the Nibelungen bristle further on.†
Chpt 1.5.2 *
This meadow was celebrated by reason of the brawls which went on there night and day; it was the hydra of the poor monks of Saint-Germain: ~quod mouachis Sancti-Germaini pratensis hydra fuit, clericis nova semper dissidiorum capita suscitantibus~.†
Chpt 2.9.1
This meadow was celebrated by reason of the brawls which went on there night and day; it was the hydra of the poor monks of Saint-Germain: ~quod mouachis Sancti-Germaini pratensis hydra fuit, clericis nova semper dissidiorum capita suscitantibus~.†
Chpt 2.9.1

Definitions:

(1) (Hydra from Greek mythology)
in Greek mythology, a many-headed serpent whose heads grew back when cut off; more generally, a difficult problem that seems to multiply when you try to solve it

In the myth, the Hydra was a monster with many heads (often said to be nine). When one head was cut off, two more would grow in its place, until Hercules finally defeated it by burning the neck stumps so new heads could not return. Today, people use Hydra or a hydra-headed problem to describe troubles with many parts or a problem that keeps coming back in new forms whenever one part is dealt with.
(2) (meaning too rare to warrant focus)