The Celtic goddess Epona was protectress of horses, asses, and mules, as well as their human caretakers. 

Epona’s feast day on the Roman calendar was December 18. The Romans embraced her as protectress of the Roman cavalry,  invoked her on behalf of the Emperor, and worshipped her as Epona Augusta or Epona Regina. She even had a shrine in the barracks of the Imperial Bodyguard.

The worship of Epona in stables is described in The Golden Ass by Marcus Apuleius.

Apuleius writes of shrines that were built for Epona, not just in temples but also in stables, presumably to protect the horses and asses in them. ♥