Before this get trashed, i saw a couples of months ago in a book of populair expressions the meaning of hold your horses, how it got started to be said etc. People used to say it long time ago. When people go with horse to some place and will take some time to go away, they should hold their horses since they wouldn't be done so fast. Bascikly this is the same as say "Not so fast/calm down" when someone wants to rush something or takes some hasty attitude (not sure if hasty is the proper word)