(To my knowledge she's not mentioned at all by Rowling.)
I can never see references to her without amusement. Bear in mind that the "canonical" Baba Yaga of Russian folklore is a cantankerous old witch who hobbles around with one leg made of bone, lives in a hut on chicken legs, and uses a giant mortar and pestle as her transport of choice. Such a character would make the likes of Hagrid and Moody seem wholly pedestrian.
Eliezer's re-imagining of her as a Dark Lady, meanwhile, just summons the most fantastic mental images.
Maybe the author has read some theory on Russian folclore. Курьи ножки might not be chicken's legs; some scholars think курьи means 'made from smoke' and so BY's hut is actually a portal between worlds (between the world of the living, where the hero comes from, and the world where Koschey the Immortal lives - Koschey, incidentally, has something of a Horcrux - a needle hidden very thoroughly, and if you break it, he dies.)
New chapter!
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 102.
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: