Her's more info about Molly and Tenbrooks:

Monroe's arrangement is based on a Kentucky folk song which is thought to date from 1884. It describes a 4 mile race held at Churchill Downs in 1878 between Molly McCarthy and Ten Broeck.

Monroe first published an arrangement of the song in 1947 with the title Ten Brooks and Molly (The Race Horse Song) but he did not record it at that time. It was however in his concert repertoire and played during radio broadcasts. It was originally arranged as a vehicle for Earl Scruggs banjo.

The Stanley Brothers version of the song was learned from Monroe concert versions by Pee Wee Lambert. It is seen by some as a pivotal record in bluegrass history. Said to be one of the first pieces of recorded evidence of the musical style moving from being that of one group (Bill Monroe and The Blue Grass Boys) to be that of a musical genre.

Versions based on Monroe's arrangement occur as Molly and Tenbrooks or occasionally Tenbrooks and Molly. A more varied group of songs based on the earlier folk song occur with a variety of names such as; Old Tim Brooks, Tim Brooks, Run Molly Run and The Race Horse Song.

Any ideas about the folk song Monroe's version is based on?