Master Pyreanor is the Paladin Trainer in Orgrimmar. He stands just to Thrall’s left side, off the raised throne, on your right entering his back chamber. Yes, folks, it appears that all the sources (and there are a lot of them, even on the official forums, although WoWWiki is reflecting a correct list) about the only Horde paladin trainers being in UC and SM are dead wrong. Here he is, in the flesh. He even trains you if you pay him some gold.
No longer will there be hundreds of Horde paladins taking the long swim across the big pond to train every 2-6 levels, depending on how anal you are.
(Thanks for the tip, Halite.)