Reference is made to a monstrous aquatic Geonosian hydra in two other SWRPG guidebooks.
First, in Coruscant and the Core Worlds, pg 15, (concerning the Royal Icqui Aquaria on Coruscant) "...the facility's interactive learning annex features [...] the skeleton of a Geonosian hydra..."
Second, in Geonosis and the Outer Rim Territories, pg 59, "The muddy bottom of the Ebon Sea is also home to gigantic hydra worms."
My theory to reconcile the hydra worm of CatCW and GatORT with the hydra dragon of Ultimate Adversaries is that the hydra worm is a larval form of the multi-headed hydra dragon. These immature aquatic hydra live in the Ebon Sea, then metamorphoise into the hydra dragons. Similar to dragonflies, I guess.