Humans can last much longer than stallions.
While a stallion is bigger, they can't fit there whole member in, they only hump about 3-4 times, the mare never cums and they can't do what human fingers can.
A human can go from 10-20 minutes, can efficiently use his fingers and tongue, and can get the mare to cum. S while the mare may not be impressed initially, she will be surprised after the first 20seconds when she sees you're still going strong, at 3 minutes, she will be in bliss, at 10+ minutes, she will be speechless and breathing hard as she tries to recover from multiple orgasms. She probably won't be able to walk for a bit too.