Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
September 29, 1948, a 2- year-old Stanradrbred stallion was admitted to Stange Memorial Clinic for treatment of an injury of the penis which had occurred the night before. The history, as offered by the owner and the groom, indicated the stallion had escaped from his box stall into a runway in the barn. A Shetland pony was kept in a box stall near that of the stallion. The groom stated that the pony had remained within the stall throughout the night and therefore could not have caused the injury. The clinicians at the clinic were of the opinion that the stallion had become excited while free and running, and erection had occurred. The constant pummeling of the erected penis against the hocks while running was probably the cause of the injury to the penis.
Towers, Charles M.
"Injury and Hematocyst of the Penis,"
Iowa State University Veterinarian: Vol. 11
, Article 13.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol11/iss2/13