This is Comanche, reputed to be the only survivor of Custer's 7th Cavalry after their fateful battle at Little Big Horn.
The plaque states:
" Comanche, the only survivor of the Custer Massacre, 1876. History of the horse and regimental orders of the 7th Cavalry as to the care of Comanche as long as he shall live 1887."
I would think that some other horses survived and were taken by the various tribes that fought there, but it does make for an interesting picture.