The hydra's claw is an item dropped by the Alchemical Hydra. It can be combined with the Zamorakian hasta to create the dragon hunter lance.
A player uses the claw on the Zamorakian hasta, turning it into a dragon hunter lance.
When attempting to create the dragon hunter lance, a warning will appear in the chatbox stating that the process cannot be reversed, and a confirmation will appear afterwards asking if players would like to proceed with the alchemical combination. Should the player confirm, the chatbox will state You successfully combine the Hydra claw and the Zamorakian hasta to create the Dragon hunter lance.
For an exhaustive list of all known sources for this item, see here