Since ponies are a polygynic species, mares are all gay, having evolved as a means to lure a stallion+please each other when one mare is busy with him. They evolved to get aroused when seeing an another one's bits and found each other pheromones attractive, as that signals "fellow herdsister". This is also why they're okay with human men despite them having nipples unlike stallions.
During fun times, a mare that just fucked you can indulge in cumsharing, tribbing with her herdsister as your cum leaks out into her.