If you have a top tier of horse and it dies, you'll sometimes get that horse back when you whistle, but you'll usually get a different horse. Mine died and was replaced by a light brown horse with black hair at the tips of its maine and tail. It has died two or three times, but still came back. I just accidentally fell of a cliff and killed it and ended up with a brown and white horse.
So I decided to go out and catch the Kentucky Saddler(which is the horse Bonny gives you). They're quite easy to find, and are one of the three fastest horses in the game. Just look for a Blonde and pearl horse in the wild. If you are unsure you caught the right one, you can take it to your hitching post and hitch it. The game will autosave, head to the nearest shop and buy the Deed for the horse in question, in this case the Kentucky Saddler, go outside and try and use the Deed from the item menu. If you have the right horse the game will tell you that you already have a horse of this type. If not, load your autosave and go hunting again.
Although be wary, if you like getting all the trophies/achievements you need to keep the same horse for 20 missions in a row.
__________________
|