Mathlann

Mathlann, the Lord of the Deeps is the High Elf and Eldar Poseidon, except he doesn't rape virgin brides on their wedding night.

Warhammer Fantasy

Mathlann likes creating sea monsters for kicks and creating storms by practicing his belly flop in the middle of the ocean. He's also a god of horses to a small extent, although it's likely because he's Kurnous's buddy rather than him creating horses in the first place. Most elves think of him as evil and a god you only try to not piss off like Khaine, but Cothique holds him as their patron god above Asuryan due to their manly belief that wrestling sea monsters bare-chested is something that should be done whenever possible, and the elves of Lothern think he's a bro since their sea monsters tend to ignore elves and their allies while consuming Dark Elf and Norsemen ships whole. While this is technically HERESY, elves from other regions just bitch and make fun of them instead of FWIPing them. So far he's saved Sea Lord Aislinn, the admiral of the Lothern fleets in peacetime and the entire High Elf fleet from all kingdoms in times of war, three times from being killed making Aislinn the closest thing Mathlann has ever had to a mortal champion. In addition, the sailors of Cothique vessels are as devoted to him as any priest. There is also a human god, Manann, who is identical to him.

40k Version

Mathlann has never been mentioned in 40k. Presumably, he perished alongside the rest of the pantheon of the Eldar via rape and consumption when Slaanesh was born. Due to the reduced importance of the sea in the GRIMDARKNESS OF THE 41ST MILLENIUM WHERE THERE'Z ONLY WAR he may have been god of outer space.