1) Well, according to my Botanica's Orchids, ed. 2002, the "mule-ear" has been moved from Oncidium to Trichocentrum. Ten species are listed as well as cultivation notes. Do you have access to the book?
2) ".... the mule ears are a great group because they grow easily and put out an abundance of flowers. The group is characterized by long, rather narrow foliage shaped very much like a mule's ear. There may be a small pseudobulb at the base of the leaf as in O. splendidum, but this is the exception. Size ranges from O. pumilum with its hoard of tiny yellow flowers to O. Mem. Pepita de Restrepo with leaves over a meter in length and spikes up to 3 meters in length. Until the popularity of equitants, O. Dr. Schragen (splendidum X lanceanum), a mule ear, was the most awarded oncidium." Source: Equitant & Mule Ear Oncidums by Ed Wright
3) Ortho's All about Orchids has a small discription in the Onc. section