Nobody can guess what TSR was thinking when they crammed these oddities into the Monster Manual. They ran out of good ideas in the first book and then wrote several more books full of this stuff.
Zack: Be honest, Steve, how many times have you included these in a game?
Steve: I think Keith might have used them one time, but it might just be a bad dream I had.
Zack: They're manta rays as smart as people that tend to be religious vampires. That is conceptually too complicated to throw out there as a random encounter.
Steve: Yeah, I guess you have to build a whole plot around these things.
Zack: You can't just roll some random encounter dice and tell your players, "Alright dudes, you're walking along the beach and you hear some praying. You look over and you see these manta rays in a church. Some of them look really pale."
Steve: "Roll initiative."
Zack: I wonder what sort of religion they follow.
Steve: They're chaotic evil, so it's gotta be a chaotic evil church.
Zack: What was the name of Ted Haggard's church?
Steve: Was he that undead half-elf priest of Vecna from Descent into the Depths of the Earth?
Zack: Yes. That's him.
Can't tell a drinking fountain from a urinal? We've got you covered. Brush up on your drinking fountain enthusiast -- or sipper -- vocabulary and learn to talk and swap sips with the best of them.
2 PM: Steven J. accidentally drops his vintage Trapper Keeper, revealing erotic drawings of the ‘bunny girls’ emoji. The room draws silent. Slowly, member after member opens his/her notebooks and tablets, revealing dozens of pages of bunny girl emoji fanart. The room votes 12-0 never to speak of this again.
Zack Parsons, Steve "Malak" Sumner, and friends tackle bizarre role playing game products that make them wonder, "What the fuck!?" From the early days of Gygax to contemporary role playing games, none will be spared.