Lord of the Rings Online isn't (yet) the game for you. That said, if Turbine's Tolkien treatment is lacking in the Rohirrim department, it doesn't lack for cool contests like the latest edition of the Design a Horse competition.
One winning player design will make it into the live game in the form of a special mount to be given away at 2011's PAX East convention, and Turbine has provided a template as well as a set of entry instructions on its official LotRO website. The news blurb also features a few examples of existing in-game equestrian skins, so don't delay in submitting your work for consideration. The fate of Gondor depends on it!