I like a little too much this morbid idea of a big ball of domesticated horses, passengers and food, slowly cutting loose bits of its horse-o-sphere to reach an absurdly far away destination. The ecological impact of the horses released to fend for themselves would be profound at the outset... But you could just go Mongolian and have the humans make jerky out of and otherwise consume the horses, for extra mileage.
Not just kill them to make jerky, but bring nursing mares along, using them to convert human-inedible plants into milk products. You can also make a small slit and drink their blood for the same effect.
