If you're hoping to see the great leviathan Lagiacrus pop up in Monster Hunter: World, feel free to exhale and keep on breathing. The big blue boy isn't destined for Capcom's latest entry in the series. Not even as DLC.
The sad news ("sad" if you enjoy getting wrecked by titanic sea life) was delivered at the packed "Monster Hunter: World Postmortem: Concept Design through Prototyping and Iteration" panel at GDC 2018. Game director Yuya Tokuda confirmed Lagiacrus isn't coming. However, the sea-beast was prototyped for Monster Hunter: World. Panel-goers even received visual confirmation Lagiacrus existed within World at one time. He was pulled, though some of the ways in which he impacts the environment was used for another water-loving monster, Jyuratodus.
It's a shame Lagiacrus was ultimately omitted from World, but that's how game development goes sometimes. Why not kill a Deviljho and make yourself feel better? Just arm yourself well before going out there.