Some information gathered from the net

What is a mule? (Our Georgina is a Hinny and therefore smaller and less agrressive than her cousins)
A mule is a hybrid cross between a male donkey on female horse or a female donkey on male horse, although the latter cross is more correctly known as a "hinny". Despite the fact that both mules and hinnies each have one horse and one donkey parent, the two crosses generally differ from each other in appearance and stature and to some extent in temperament - a fact which has been recognized since they were first bred.
Mules, having hybrid vigor, can grow taller than both parents. Weight for weight they are stronger than horses, and are much longer-lived with much longer working lives, although maturing slightly later. They rarely become lame or ill or suffer wounds, can withstand extremes of temperature, can live on frugal rations, have tremendous stamina and resilience and are exceptionally sure-footed.