Lagiacrus

Gilkura is an unconfirmed monster that will be featured in Monster Hunter 3: Tri. It is skinny and serpentine, and swims gracefully in waters. Its identity is very secret, and its only known existance comes from the end of the MH3 trailer. It also appears to make a whale-like noise.

Rumors
Gilkura is apparently going to be the 4th Fatalis, and Capcom is creating a new fighting style for this monster, especially for this quest. Other than these theories, not much is speculated. A creature surrounded with mystery...