Indeed, all of the former Flagships were treated poorly in any newer games, not given their respected subspecies either. Lagiacrus was a creature born of the MH3 and MH3U games, also much to my knowledge, he was one of the first actual Leviathans, since he's got such a scary sea dragon look. Lagiacrus had a subspecies and a whole practical Deviant class before Deviants where a thing. Ivory and Abyssal Lagiacrus. Ivory Lagiacrus prioritized land battles over water battles (those were a thing in 3 & 3U) and had deadly electric abilities and realeased gigantic electric shockwaves. Abyssal Lagiacrus was a complete underwater battle fought in the Ceadeus dome where a lost civilzation is located. It's an extremely deadly fight since Abyssal has super electric capabilities and can harness the underwater currents to make little electric whirlpools that can KILL.
Lagiacrus comes back in MHGen in a rather crippled form, his power and stance has moved a lot more in the direction of Ivory, however for some odd reason, Ivory isn't included in MHGen. Lagiacrus always looked like a crocodile to me to be honest. The ecology behind Lagiacrus is that he sits in the sun to gain solar power, which he turns into electricity that is formed from the spikes on his back. He can puff balls of raw electricty from his mouth to shoot at you, some of the balls have the potential to explode into whole bursts of electricity that have a huge radius of damage. Normally, he would enrage by charging himself which he reels inwards and shoots back out releasing balls of electricity to circle around him and a mild burst forms around his body. Normally, it was an extremely dangerous attack in 3U when it was underwater.