I think the horse was just a dlc pack for preordering. The expansions added around 6-10 hours each for their main story alone. Side quests and exploration are always ridiculous in the TES games though.
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According to the Wiki the first DLC pack for Oblivion was Horse Armor. It cost $2.00 and allowed you to change what your horse looked like in game. There is no functional change in the game except what your horse looks like. Also I will bet that people play more that 15 hours on Crota's end alone, but that is just speculation.
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Just $2 though, and I bet not a single person who didn't buy it was hampered from their game in any way shape or form from playing the main game. Bungie did not extend that courtesy. I'm not bitching there cuz I have the DLC, just making a point