No Yes they can, my mare produced milk last year (she has never had a foal). Some plants they eat can mimic hormones that stimulate milk production. If you are worried call your vet,cushings disease...
If a mare shows signs of lactation (bagging up with milk) she is most likely in foal and within 30 days of having her baby. Some mares have hormone imbalances that can cause them to lactate, but this...