Again, this odd expression is listed by Heraeus in his "Index graecolatinus" (1903) to the medieval Corpus Glossariorum Latinorum, again without reference or explanation. It seems highly unlikely to go back to a coherent ancient source, since it means literally "charioteer-horse," and a horse can't be its own charioteer. The τροχηλάτης has to be a human charioteer. More reasonable is the Lewis & Short reference to currilis equus as equivalent to Greek σὺν ἅρματι ἀγωνιζόμενος ἵππος, "horse contending with a chariot."