Methinks you are forgetting where D&D started from - modelling Tolkien. None of those critters existed in OD&D. But races/classes and creatures were added because that's what people wanted.
Part of fantasy is creating what we - as creators (ie: DMs) want to experience. If I want to experience a ... frig, what was that stupid movie with the blue elves ... Avatar universe, I can do that. If I want to go full-bore Tolkien, I can go there. It's up to me to tell people that I'm loading up the planet with tree-hugging blue elves, or bitchy dwarves and immortal elves. They don't wanna play in that world, there are two options: change the world, or change the players.
Hell in my last campaign, I had Sea Giants because damnit, I liked Stephen Donaldson's work.
I read another novel where horse-mounted plainspeople - Cagmae? - existed. I had those in my last setting. I also had Vulcans of a sort....
This time, I have only retained the horsemounted plainspeople, treating them as expelled Scottish/Irish elements who adapted to a different climate.
This time around, my Sylvan elves are essentially Japanese. I had a general rule that PCs could not be Ronin Samurai without a damned good reason, and one of my players created a ronin Sylvan elf Samurai with a damned fine backstory... but a backstory that fit into the world. The player compromised, but so did I.