When using the rampage trainer, I’m able to spawn a variety of coats of each horse breed but a lot of them are from online (when spawned the “coat” section when looking at the horse stats  is blank and when I save the game with one of them as my main horse they aren’t there when I load into the game. Is there a way to naturally implement them into the game?  For example, the chestnut coat of the Turkoman looks beautiful but when I save the game with it as my saddle horse the next time I load into the game it changes into a Tennessee walker. It’s as if the game can’t recognize it.

there are mods on Nexus that allows you to replace storymode horses with online horses. Go here https://www.nexusmods.com/reddeadredemption2/mods/598 and https://www.nexusmods.com/reddeadredemption2/mods/714 and https://www.nexusmods.com/reddeadredemption2/mods/889  also this one has the chestnut turkoman coat https://www.nexusmods.com/reddeadredemption2/mods/815 

i hope this helps you, currently you cant add them in you can only replace the coats on existing horses in the game. you need lennys mod loader to use this mod. it will have instructions on how to install and use it. Best of luck!