The roan's owner, tired of it siring equally stubborn offspring, decides to put an end to the matter by gelding the beast. They rope it down, and a cowboy commences the operation. Before it can be completed, the roan bites off the owner's own equipment
The story of this song apparently begins in a fit of pique.
According to Logsdon, Curley Fletcher wrote the original "Strawberry Roan" as a poem, to which a melody was later added. But Fletcher the chorus/bridge, which was the work of Fred Howard and Nat Vincent. So he produced this extremely scatological parody to get back at them.
How traditional it is is an open question. The Sons of the Pioneers recorded it, anonymously, and Baxter Black sang Logsdon a variant on that. Legman, on p. 404 of _The Horn Book_, considers it one of the few genuine songs on a "private party" 78. But I suspect the Sons of the Pioneers recording is the source for nearly all of the few versions collected.
Logsdon's entry on this song includes two other Strawberrry Roan variants that could not be sung in polite society. One was simply a more detailed saga of riding the roan; the other is about a visit to a whorehouse and is basically "Kathusalem (Kafoozelum) (II)" adapted to cowboy circumstances. These two might be traditional, but until I find additional collections, I'm merely going to note them.