My understanding from passing conversations is this:
Horses have been left out of DTV because they made the mode far too easy (I assume by kiting). This is easy to believe if you've seen a player grab a horse during Nomad or Rohirric early enough in the wave where they can still grab all the aggro.
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So my suggestion is this:
Allow horses but by their health drains as they are used, akin to fuel.
The exact rate of decay and the factors in the formula are up to chadz really, but here are some factors to consider:
HP drain is 100% linear, making the slower, more expensive horses always last longer (balanced by their cost)
or
HP drain is based on horse HP minus its armor value (probably more balanced this way)
or
One or Both of the previous two ideas, with Riding ability further reducing HP consumption.