Thanks for that information. I had heard that Demeter was sometimes referred to as mare-headed. However, it turns out that the allusion toa mare-headed goddes in the Golden Ass is entirely a fanciful embellishment in the Graves translation. Other translations such as Walsh do not have it, and it is not in the original Latin.

I quoted the Walsh translation instead of the Graves one on the EPONA.net page.

Regarding the fasti, please tell me more (ideally with a reference so I can follow this up). The only reference to Epona in a roman calendar that I am aware of is the one from Guidizzolo (Mantova, Lombardia, Italy) which is south-west of Verona and north-west of Mantua. This is, as you say, in Cisalpine Gaul. The calendar is now in the church of St. Martini. It was first published in Revue Archeologique, 1892, II p146 and is also given in Dessau, Inscriptiones Latinae Selectae, Ferialia. ILS 4917

The actual inscription is as follows:

................    
.... XIII  XV  III IDVS IVL  
.... XII  XIIII  APOLLINAR  
.... XI  XIII  X K AVG  
.... X  XII  NEPTVNAL  
.... VIIII  XI  IDIBVS AVG  
.... VIII  X  DIANA  
.... VII  VIIII  X K SEPTEMBR  
.... VI  VIII  VOLKANALIA  
.... V  VII  III IDVS DECEMBR  
....    VI  SEPTIMONTIVM  
.... IIII  V  XVI K IANVAR  
....    IIII  SATVRNALIA  
.... III  III  XV K IANVAR  
.... PRID  PRID  EPONE