Lagiacrus was first teased in the showcase for the Free Title Update 1, which will be arriving April 4th.
At the end very end of the video, we were shown a slide for the Second Free Title Update, accompanied by water sounds, a distinct roar, and the claw below:
By putting the pieces together, we can identify the monster as Lagiacrus, who originally appeared in Monster Hunter 3 (Monster Hunter Tri).
Lagiacrus is a Sea Wyvern, also known as "Lord of the Seas" or "The Azure Lightning".
In its MH3 fight, you fought it underwater, so it will be interesting to see what mechanics it will have in Monster Hunter Wilds.