In Minecaft (vanilla), is it possible for horses to have more than 15 hearts? The Minecraft Wiki says that is the max, but everyone I have talked to (as well as a faint memory I have from a while back) says horses can have more than 15 hearts.
No, a horse's base health cannot exceed 15 hearts. Well, except for some special circumstances. For one, it's obviously rather easily modifiable through editing and modding.
But even in vanilla Minecraft, it's theoretically possible to raise a horse's health above the maximum: The Health Boost and Absorption Status Effects. Both of these grant extra hearts that will vanish once the effect expires or is countered. (The Health Boost effect is currently not accessible, however.)
I know this is late, but just in case anyone else stumbles upon this thread, I have decided to give some information. No horse (whether spawned with egg or wild spawn) can have 15 hearts. The only way to get to the maximum capacity of hearts is to breed it with a horse who had also a high health bar. Soon after correct multiple breeding processes', you might have a horse wit h15 hearts.