Unpack the horse file(s) you want
Rename the file(s) to the horse(s) you want to replace
Copy files to 'RampageFiles\Streaming' or 'lml\stream' folder

- Here's an example list of potential horse file names
a_c_horse_americanstandardbred_black
a_c_horse_americanstandardbred_buckskin
a_c_horse_americanstandardbred_palominodapple
a_c_horse_americanstandardbred_silvertailbuckskin
a_c_horse_appaloosa_blanket
a_c_horse_appaloosa_brownleopard
a_c_horse_appaloosa_fewspotted_pc
a_c_horse_appaloosa_leopard
a_c_horse_appaloosa_leopardblanket
a_c_horse_arabian_black
a_c_horse_arabian_redchestnut_pc
a_c_horse_arabian_rosegreybay
a_c_horse_arabian_warpedbrindle_pc
a_c_horse_arabian_white
a_c_horse_hungarianhalfbred_darkdapplegrey
a_c_horse_hungarianhalfbred_flaxenchestnut
a_c_horse_hungarianhalfbred_piebaldtobiano
a_c_horse_missourifoxtrotter_amberchampagne
a_c_horse_missourifoxtrotter_silverdapplepinto
a_c_horse_mustang_grullodun
a_c_horse_mustang_tigerstripedbay
a_c_horse_mustang_wildbay
a_c_horse_turkoman_darkbay
a_c_horse_turkoman_gold
a_c_horse_turkoman_silver

For example rename 'a_c_horse_missourifoxtrotter_sablechampagne' to 'a_c_horse_missourifoxtrotter_amberchampagne'
then copy it to your Streaming or Stream folder to replace Amber Champagne with the unique and unattainable Sable Champagne.

Note: If you do not rename the files and just copy them, they won't do anything unless you spawn the horses with a trainer.