Here's why:
* Chromosomal Differences: Horses have 64 chromosomes, while donkeys have 62. This difference makes it difficult for their offspring to have a complete set of chromosomes.
* Fertility Issues: Mules and hinnies are typically sterile because of the uneven number of chromosomes. However, there have been rare cases of fertile mules.
* Offspring Viability: The offspring of a mule and a hinny are usually weak and often don't survive long.
Therefore, while theoretically possible, the offspring of a mule and a hinny is a highly unusual and often short-lived event.