There was a cavalry depot, Geisboro, outside of Washington, DC- I'm unclear exactly how it functioned except I've seen it listed under ' remounts ' in indexes. SO massive! Brady did a series of era photographs. I'm also not clear on which would be the best references for Geisboro? Era papers tend to be excellent- official reports, etc. were printed as these war time resources were put in place.
This site seems pretty awesome- title states " Parking For 30,000 Horses ". It did not serve up all the horses needed, guessing- but wow, a
lot.
"
The area once known as Giesboro Point is now occupied by the Department of Defense's Joint Base Bolling-Anacostia. The installation's major tenant organization, the Defense Intelligence Agency, is observing its 50th anniversary this year. However, nearly 150 years ago, this parcel of land was not a military intelligence headquarters, but the logistics hub of the Union Army's cavalry in the Eastern Theater. By providing a ready supply of mounts to the Army of the Potomac, Giesboro Cavalry Depot made an important contribution as the army slugged it out in Virginia. Over 200,000 horses were received, issued, died or sold at Giesboro during the war. "
http://civilwarwashingtondc1861-186.../geisborough-point-cavalry-depot-parking.html