Horses Would add a Great mode of transportation to destiny.In addition to being slow as -blam!-,they could be expensive aswell.
Higher end horses would have the ability to have guns strapped to them.you could buy armour and weapons for these horses.
you would be able to use you weapons on horses.You could strap a bomb to a horse and use it as a suicide-horse,
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Eff that lets go bigger, why not dun dun dun my little ponies! There's even a shader called sparkle pony or some crap like that, have seven or so for each class and make them exotics, you could ride them rodeo style in the tower or out on patrol, he'll let it rain stars, fireworks and rainbows, just think about it...epic I know ;)