there are certain changes that all horses undergo.  they are minimal however, and do very little to mute creativity.  these are natural things that happen when a horse dies. 

for one, the entire eye becomes one of three colors.  white, black or red, and has a slight glow to them.  this is true for any animal that comes to the after life... be it bird or wolf or horse.  what color their eyes are usually divides them into personality types,  such as horses with white eyes might be considered innocent, while black eyes might be considered evil.  this however is not always the case.  the eye color means nothing about the horse, you must look at the rest of their bodies to see.

secondary, all of them take on a slightly thinner body tone.  not emaciated, but clearly dead in appearance.

otherwise, the sky is the limit, but the outer body must match the inner heart.  a horse with a free soul might have wings in the after life, so that no fence could bind them in, a horse with an evil heart might have horns, or spikes on their body, or fire coming from their mane and tail, while a horse with a pure heart might turn into a unicorn when they cross over.  some horses, nothing change, they appear simply as thin horses with glowing eyes. 

it is up to the creative mind of the player what it is that changes after death.

((notes, though they might metamorph  physically, they do not develop powers beyond that.  there is no elemental control, mind control or the like... though there are few and we mean very few that have learned to see beyond sight.))