Lagiacrus

The Lagiacrus is the flagship Monster of Monster Hunter 3, which it debuted from. It is a greatly feared Leviathan known as the Lord of the Seas that specializes in using specialized plates on its back to charge and electrify its enemies and prey. One particular Lagiacrus acts as the archenemy of the Moga Village Hunter and serves as the secondary antagonist of Monster Hunter 3 and Ultimate's main campaign. Though initially appearing as the main antagonist as the source of Moga's earthquakes, this turned out to be a false lead, with Ceadeus as the main antagonist.

Attributes
The Lagiacrus is a Leviathan known as the Lord of the Seas. It is serpentine in appearance, with gray-blue scales, orange eyes which glow when enraged, and a series of dermal spikes on its back. These plates serve the purpose of charging up the Lagiacrus, eventually to the point of glowing blue, so that it can release a burst of electricity to paralyze and kill its foes. The Lagiacrus is mainly aquatic, though it breaches the surface to cross to other bodies of water and to rest.

The Lagiacrus of the main campaign is first blamed for a series of violent earthquakes that disrupt Moga Village. Appearing in an early quest, the Lagiacrus ambushes the currently-rookie Hunter in an early quest. It cannot be killed in this state, although it can be repelled. The Hunter trains with the purpose of defeating the Lagiacrus and returning order to Moga. Confronting the Lagiacrus a second time, the Hunter repels the Lagiacrus, and finally concludes the battle by either killing or capturing the Lagiacrus. However, it turns out the Lagiacrus was not the cause of the earthquakes, as revealed by Cha-Cha spotting the Ceadeus in deep waters scraping its horn against the island's bottom to alleviate pain from the horn overgrowing its eye. Far too late to make amends, the Lagiacrus ends up hunted for little cause.

Ivory Lagiacrus
The Ivory Lagiacrus is a subspecies of Lagiacrus that appears near the end of Monster Hunter 3 Ultimate's main campaign as the final boss (if one excludes Alatreon) of the main story. The Ivory Lagiacrus has much more potent electricity, and has ivory-white scales, hence its name. It is also much more land-focused, very rarely venturing into bodies of water. The Ivory Lagiacrus fought as the final boss was the arch-enemy of Moga Village's chief, who fought against it for years until he was finally forced to retire. With its reemergence, the Hunter is able to avenge the village chief and defeat the old Ivory Lagiacrus for good.

Abyssal Lagiacrus
The Abyssal Lagiacrus is a bonus boss fought in Port Tanzia's G-Rank quests (or as a downloadable, HR8 G-Rank quest). The Abyssal Lagiacrus is a much rarer version of the Lagiacrus that dwells in extremely deep bodies of water. It is jet-black, and much more powerful than the regular Lagiacrus, able to swim fast enough to create underwater currents. It is only recounted in an old scroll:

From the abyss' darkest cave

Comes the master of the wave

Consumer of the world entire

Devil of the sea most dire

Fear it as you fear the grave

Very little is known about the Abyssal Lagiacrus, and it takes residence in the deep where Ceadeus and Goldbeard Ceadeus are fought.

Trivia

 * The Abyssal Lagiacrus is able to be easily defeated with a team of Gunners by exploiting a glitch that involves hiding between a rock and firing at the Abyssal Lagiacrus, who due to the glitch is unable to follow into the rock.