I'm wondering where Odieyama got the transcript of the story of "Little Dark Pony," and where the story originated.

Odieyama answered the dark pony question in November, 2010, over five years ago. Odieyama is no longer posting on this site. Call or visit your local library to ask about Dark Pony and Sleepy Town. I am a retired librarian and will also check and report back in this thread.

It's a folk tale and was added to an elementary school reader, copyright 1946. Here's a link to a blog post and comments about it:

Love During Wartime

Here's another (better!) link:

Dark Pony Designs: Dark Pony: The Original Folk Tale

Apparently, there are still used copies of that reader for sale. Amazon indie vendors may have copies available, plus biblio does:

The New Our New Friends & Jane 1951 Dark Pony / Sleepy Town by William S Gray; A Sterl Artley; May Hill Arbuthnot; Keith Ward [Illustrator]; Evelyn Copelman [Illustrator]; - 1951

Be sure, if you purchase a copy, it's the correct one that contains the "Dark Pony" story.