I'm pretty sure that 6 years part can't be true.
A.M.M. [Army Medical Museum] No. 10156
March 9, 1891
Robinson Dr. C.B.
Foetal bones, said to have been discharged from the uterus of a mare, about 12 years old. Owned by Senator J.S. Barbour of Virginia.
It is stated that she had not been put to a horse for 6 years.
History received verbally
Specimen received Mar. 8, 1891