Alright. So I was trying to make a zombie horse with chests (leather pouches), diamond armor, and a saddle. first. I made a horse. I put a saddle and diamond armor on it. Next, I made it into a donkey with /entitydata and put the chest on it. Then, I made it into a zombie horse (Type:3) then it's pure white. even the saddle and stuff
zombie/skeleton horses aren't supported
also they cannot wear armor or chests