Shyssiryxius, the Female Great Water Dragon is Blup's mommy and finding her can help complete the finding Blup's mommy quest. She is found in the hidden water cave. This can be found in the lake northwest of Terinyo and you may need to search for it. Shyssiryxius is also the guardian of a great horde of treasure.
Killing Shyssiryxius can be very benefitial, especially for chaotic players who do not recieve any downfall. This is only recommended for high level players. Do not attempt to steal any of the great water dragon's treasure for she will immediately become hostile, although (unconfirmed) it is possible to use a scroll or the spell of calm monster to calm her and get away with her treasure (unconfirmed). Your best bet is to get a very high DV and evade and attack Shyssiryxius from a distance with the hope that her water breath attack will not hit or at least not kill you.