ice legendary?

the number 158 monsters any clue on how to get him? i alredy check shops, boss rewards, quests etc no clue, i know its kinda fast to ask but i wanna know if anyone has any info on him.... theres a posibility he will be the boss of Ocean Abyss 15 expert

[Koenaj]

I saw him aswell, but I too have no clue on how to get it.
It might be the Phoenix-like monster, unable to catch, but can be encountered as Boss Raid/Veteran or Expert boss.