Some text from Minecraft Wiki used under Creative Commons Attribution-Share Alike License 3.0
There are three types of horse armor, those being iron, gold and diamond. Horse armor does not break over time like most tools. Even though gold is the worst in tools because of its low durability, gold horse armor is actually a bit better than iron horse armor, it provides more protection and is much rarer. Diamond is still the best for protection when it comes to horse armor.
Horse armor can be found naturally in dungeon chests, blacksmith chests, Nether Fortress chests, and Abandoned Mineshafts .
Iron horse armor- 20%
Gold armor- 28%
Diamond horse armor- 44%