Finally, the hydra is finished and uploaded to the Neverwinter Vault and the Neverwinter Nexus. I made a tintable and non-tintable version of the model, as can be seen below.
I'll rig and upload the one-headed version soon. For now, I need sleep. Zzzzz.
Edit, 29 May 2012. Some time ago, I posted a thread regarding this creature in the NWN2 Custom Content forum at the Bioware Boards. When I announced the release of the hydra, a few people started trying it out. As of this writing, MokahTGS posted a test module with hydra variants in it for other players to try and contribute modifications to. Anyone interested may head over to my Big Snake thread to apprise themselves of the latest developments and perhaps contribute to improving the creature's blueprint.