Equus conversidens
Equus conversidens, or the Mexican horse, is a dubious Pleistocene species of horse, now extinct, that inhabited North America. The holotype of "Equus conversidens", a partial palate, was unearthed in Pleistocene deposits northeast of Mexico City, Mexico. In January 1963, a partial skeleton was found in the city of Canyon, Texas in a white clay bed during the excavation of a basement, and was referred to "E. conversidens" by Dalquest and Hughes (1965), who interpreted the species as medium to small-sized, and added additional records of the species from Texas (including a skeleton from Slaton), Arizona, New  Mexico, Oklahoma, Kansas and Florida, synonymizing "Equus francisci", "Equus tau", "E. littoralis", "E. achates", and "E. barcenaei" with "E. conversidens". However, Winans (1985) and MacFadden (1992) challenged the validity of "E. conversidens" due to its minimal diagnostic value, and treated "E. francisci" as valid. Subsequent study confirmed the "nomen dubium" status of "E. conversidens" because the holotype was undiagnostic besides its small size.