remuda
noun
re·mu·da ri-ˈmü-də 
-ˈmyü-
: the herd of horses from which those to be used for the day are chosen
Word History
Etymology
American Spanish, relay of horses, from Spanish, exchange, from remudar to exchange, from re- + mudar to change, from Latin mutare — more at mutable
First Known Use
circa 1892, in the meaning defined above