This are the codes you need to rename your .ymt files for story mode horses. 
You only rename .ymt files, NOT .ytd.
By doing this, you replace a horse in your game with my new modded horse (for example, naming one of my files to a_c_horse_arabian_white will make your new modded horse to appear instead of the vanilla white arabian). 
You can change the name of the one I gave you by whatever of these names. 
It doesn't matter if it's upper or lower case, just write what comes before the .ymt file extension right.

American Paint Horse:
A_C_Horse_AmericanPaint_Greyovero	
A_C_Horse_AmericanPaint_Overo	
A_C_Horse_AmericanPaint_SplashedWhite	
A_C_Horse_AmericanPaint_Tobiano	

American Standardbred:
A_C_Horse_AmericanStandardbred_Black	
A_C_Horse_AmericanStandardbred_Buckskin	
A_C_Horse_AmericanStandardbred_PalominoDapple			
A_C_Horse_AmericanStandardbred_SilverTailBuckskin

Andalusian:			
A_C_Horse_Andalusian_DarkBay			
A_C_Horse_Andalusian_Perlino				
A_C_Horse_Andalusian_RoseGray

Appaloosa:			
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

Arabian:	
A_C_Horse_Arabian_Black	
A_C_Horse_Arabian_RedChestnut	
A_C_Horse_Arabian_RedChestnut_PC	
A_C_Horse_Arabian_RoseGreyBay	
A_C_Horse_Arabian_WarpedBrindle_PC
A_C_Horse_Arabian_White	

Ardennes:
A_C_Horse_Ardennes_BayRoan	
A_C_Horse_Ardennes_IronGreyRoan	
A_C_Horse_Ardennes_StrawberryRoan
	
Dutch Warmblood:
A_C_Horse_DutchWarmblood_ChocolateRoan	
A_C_Horse_DutchWarmblood_SealBrown	
A_C_Horse_DutchWarmblood_SootyBuckskin	

Gang Horses:
A_C_Horse_EagleFlies	
A_C_Horse_Gang_Arthur	
A_C_Horse_Gang_Bill	
A_C_Horse_Gang_Charles_EndlessSummer	
A_C_Horse_Gang_Hosea	
A_C_Horse_Gang_Javier	
A_C_Horse_Gang_John	
A_C_Horse_Gang_Karen	
A_C_Horse_Gang_Kieran	
A_C_Horse_Gang_Sadie	
A_C_Horse_Gang_Sadie_EndlessSummer	
A_C_Horse_Gang_Sean	
A_C_Horse_Gang_Trelawney	
A_C_Horse_Gang_Uncle	
A_C_Horse_Gang_Uncle_EndlessSummer
A_C_Horse_John_EndlessSummer
	
Hungarian Halfbred:
A_C_Horse_HungarianHalfbred_DarkDappleGrey	
A_C_Horse_HungarianHalfbred_FlaxenChestnut		
A_C_Horse_HungarianHalfbred_PiebaldTobiano	
	
Kentucky Saddler:
A_C_Horse_KentuckySaddle_Black	
A_C_Horse_KentuckySaddle_ButterMilkBuckskin_PC	
A_C_Horse_KentuckySaddle_ChestnutPinto	
A_C_Horse_KentuckySaddle_Grey	
A_C_Horse_KentuckySaddle_SilverBay

Missouri Fox Trotter:	
A_C_Horse_MissouriFoxTrotter_AmberChampagne	
A_C_Horse_MissouriFoxTrotter_SilverDapplePinto

Morgan:	
A_C_Horse_Morgan_Bay	
A_C_Horse_Morgan_BayRoan	
A_C_Horse_Morgan_FlaxenChestnut	
A_C_Horse_Morgan_LiverChestnut_PC	
A_C_Horse_Morgan_Palomino
				
Mustang:	
A_C_Horse_Mustang_GrulloDun				
A_C_Horse_Mustang_TigerStripedBay	
A_C_Horse_Mustang_WildBay

Nokota:			
A_C_Horse_Nokota_BlueRoan			
A_C_Horse_Nokota_ReverseDappleRoan			
A_C_Horse_Nokota_WhiteRoan	

Shire:		
A_C_Horse_Shire_DarkBay	
A_C_Horse_Shire_LightGrey			
A_C_Horse_Shire_RavenBlack
	
Suffolk Punch:		
A_C_Horse_SuffolkPunch_RedChestnut		
A_C_Horse_SuffolkPunch_Sorrel
	
Tennessee Walker:	
A_C_Horse_TennesseeWalker_BlackRabicano			
A_C_Horse_TennesseeWalker_Chestnut			
A_C_Horse_TennesseeWalker_DappleBay			
A_C_Horse_TennesseeWalker_FlaxenRoan			
A_C_Horse_TennesseeWalker_GoldPalomino_PC			
A_C_Horse_TennesseeWalker_MahoganyBay			
A_C_Horse_TennesseeWalker_RedRoan
			

Thoroughbred:
A_C_Horse_Thoroughbred_BloodBay	
A_C_Horse_Thoroughbred_Brindle
A_C_Horse_Thoroughbred_DappleGrey
A_C_Horse_Thoroughbred_ReverseDappleBlack
		
Turkoman:	
A_C_Horse_Turkoman_DarkBay		
A_C_Horse_Turkoman_Gold	Animal			
A_C_Horse_Turkoman_Silver		

Other:
A_C_Horse_MurfreeBrood_Mange_01	
A_C_Horse_MurfreeBrood_Mange_02	
A_C_Horse_MurfreeBrood_Mange_03		
A_C_HorseMule_01		
A_C_HorseMulePainted_01
A_C_Horse_Buell_WarVets	