Kuo-toa
Kuo-Toa are Dungeons & Dragons fish people living in deep seas and underground caverns. Former slaves of illithids and declared "kill on sight" by drow, they are completely insane from having their brains scrambled by slaver octopi and worship whatever seems the most awe-inspiring for them.
Other traits include being able to feel the presence of invisible or ethereal creatures and love for armaments that capture rather than kill. Their character-classes tend to fighter, monk ("Monitor"), and cleric.
Their society is theocratic - archbishop rules, children of archbishop ("hooks") inherit the power after they fight each other to the death. Their goddess is Blibdoolpoolp, who actually has humanoid (mammalian - and how!) characteristics rather than kuo-toa. They were introduced in module D2, living in / around her shrine in the Greyhawk setting. They got a larger city in the second act of Night Below.
They're basically the Dungeons & Dragons knock-off version of Deep Ones. They're less menacing, though, especially after the second Monster Manual filled that the nightmare sea abomination niche. They're not even as scary as the kopru in Isle of Dread.
Later editions take a leaf from Terry Pratchett and have them make a thing they believe in come true - this way, they create minor gods from whatever they start worshipping. This was how Blibdoolpoolp, made from a broken statue of a woman and crayfish parts, came into being.